PRACTICAL HINTS, ETC.*

ON THE MANAGEMENT OF DRAWING PAPER.

THE first thing to be done, preparatory to the commencement of a drawing, is to stretch the paper evenly upon the smooth and flat surface of a drawing board. The edges of the paper should first be cut straight, and, as nearly as possible, at right angles with each other; also the sheet should be so much larger than the intended drawing and its margin, as to admit of being afterwards cut from the board, leaving the border by which it is attached thereto by glue or paste, as we shall next explain.

The paper must first be thoroughly and equally damped with a sponge and clean water, on the opposite side from that on which the drawing is to be made. When the paper absorbs the water, which may be seen by the wetted side becoming dim, as its surface is viewed slantwise against the light, it is to be laid on the drawing board with the wetted side downwards, and placed so that its edges may be nearly parallel with those of the board; otherwise, in using a T square, an inconvenience may be experienced. This done, lay a straight flat ruler on the paper, with its edge parallel to, and about half an inch from, one of its edges. The ruler must now be held firm, while the said projecting half inch of paper is turned up along its edge; then, a piece of solid glue (common glue will answer the purpose), having its edge partially dissolved by holding it in boiling water for a few seconds, must be passed once or twice along the turned edge of the paper, after which, this glued border must be again laid flat by sliding the rule over it, and, the ruler being pressed down upon it, the edge of the paper will adhere to the board. If sufficient glue has been applied, the ruler may be removed directly, and the edge finally rubbed down by an ivory bookknife, or any clean polished substance at hand, which will then firmly cement the paper to the board. Another but adjoining edge of the paper must, next, be acted upon in like manner, and then the remaining edges in succession; we say the adjoining edges, because we have occasionally observed that, when the opposite and parallel edges have been laid down first, without continuing the process progressively round the board, a greater degree of care is required to prevent undulations in the paper as it dries.

Extracted from a Treatise on Drawing Instruments by F. W. Simms, Civil Engineer and Surveyor.

Sometimes strong paste is used instead of glue; but, as this takes a longer time to set, it is usual to wet the paper also on the upper surface to within an inch of the paste mark, care being taken not to rub or injure the surface in the process. The wetting of the paper in either case is for the purpose of expanding it; and the edges being fixed to the board. in its enlarged state, act as stretchers upon the paper, while it contracts in drying, which it should be allowed to do gradually. All creases or undulations by this means disappear from the surface, and it forms a smooth plane to receive the drawing.

MOUNTING PAPER AND DRAWINGS, VARNISHING, ETC.

In mounting paper upon canvas, the latter should be well stretched upon a smooth flat surface, being damped for that purpose, and its edges glued down as was recommended in stretching drawing paper. Then with a brush spread strong paste upon the canvas, beating it in till the grain of the can vas be all filled up; for this; when dry, will prevent the canvas from shrinking when subsequently removed; and, having cut the edges of the paper straight, paste one side of every sheet, and lay them upon the canvas, sheet by sheet, overlapping each other a small quantity. If the drawing paper is strong, it is best to let every sheet lie five or six minutes after the paste is put on it; for, as the paste soaks in, the paper will stretch, and may be better spread smooth upon the canvas; whereas, if it be laid on before the paste has moistened the paper, it will stretch afterwards and rise in blisters when laid upon the canvas. The paper should not be cut off from its extended position till thoroughly dry; and the drying should not be hastened, but gradually take place in a dry room, if time permit; if not, the paper may be exposed to the sun, unless in the winter season, when the help of a fire is necessary, care being had that it is not placed too near a scorching heat.

In joining two sheets of paper together by overlapping, it

is necessary, in order to make a neat joint, to feather edge each sheet; this is done by carefully cutting with a knife half way through the paper near the edges, and on the sides, which are to overlap each other; then strip off a feather-edged slip from each, which, being done dexterously, the edges will form a very neat and efficient joint when put together.

The following method of mounting and varnishing drawings or prints was communicated some years ago by Mr. Peacock, an artist of Dublin. Stretch a piece of linen on a frame, to which give a coat of isinglass or common size. Paste the back of the drawing, leave it to soak, and then lay it on the linen. When dry, give it at least four coats of well-made isinglass size, allowing it to dry between each coat Take Canada balsam diluted with the best oil of turpentine, and with a clean brush give it a full flowing coat

GENERAL RULES APPLICABLE IN ALL GEOMETRICAL

CONSTRUCTIONS.

In selecting black-lead pencils for use, it may be remarked that they ought not to be very soft, nor so hard that their traces cannot be easily erased by the India rubber. Great care should be taken, in the pencilling, that an accurate outline be drawn; the pencil marks should be distinct yet not heavy, and the use of the rubber should be avoided as much as possible, for its frequent application ruffles the surface of the paper, and will destroy the good effect of shading or colouring, if any is afterwards to be applied.

The following seven useful rules are taken from Mr. Thomas Bradley's valuable work on Practical Geometry:

"1. Arcs of circles, or right lines by which an important point is to be found, should never intersect each other very obliquely, or at an angle of less than 15 or 20 degrees; and, if this cannot be avoided, some other proceeding should be had recourse to, to define the point more precisely.

"2. When one arc of a circle is described, and a point in it is to be determined by the intersection of another arc, this latter need not be drawn at all, but only the point marked off on the first, as it is always desirable to avoid the drawing of unnecessary lines. The same observation applies to a point to be determined on one straight line by the intersection of another.

"3. Whenever the compasses can be used in any part of a construction, or to construct the whole problem, they are to be preferred to the rule, unless the process is much more circuitous, or unless the first rule (above) forbids.

"4. A right line should never be obtained by the prolongation of a very short one, unless some point in that prolongation is first found by some other means, especially in any essential part of a problem.

"5. The larger the scale on which any problem, or any part of one, is con structed, the less liable is the result to error; hence all angles should be set off on the largest circles which circumstances will admit of being described,

and the largest radius should be taken to describe the arcs by which a point is to be found through which a right line is to be drawn; and the greater attention is to be paid to this rule, in proportion as that step of the problem under consideration is conducive to the correctness of the final result.

"6. All lines, perpendicular or parallel to another, should be drawn long enough at once, to obviate the necessity of producing them.

"7. Whenever a line is required to be drawn to a point, in order to insure the coincidence of them, it is better to commence the line from the point; and if the line is to pass through two points, before drawing it the pencil should be moved along the rule, so as to ascertain whether the line will, when drawn, pass through them both. Thus, if several radii to a circle were required to pass through any number of points respectively, the lines should De begun from the center of the circle; any error being more obvious when several lines meet in a point.

PART II.-ON OPTICAL INSTRUMENTS.

UNDER this head our principal object will be to consider the construction, and principles of action, of such instruments as are indispensable to assist the vision in making observations upon distant objects, whether upon terrestrial objects for the purposes of the surveyor, or upon celestial objects for the purposes of astronomy and navigation. We propose, however, to add a few words upon such other optical instruments, as by their utility, or by the frequency with which they are brought before us, appear to demand our attention.

We shall thus be led, in the first place, to review briefly the properties of prisms, lenses, and plane and curvilinear reflectors, and shall then proceed to give descriptions of the following instruments, viz.,

Microscopes.
Telescopes.

The Camera Lucida.

The Camera Obscura.

Such as are adapted to surveying and astronomical instruments, rather fully.

Very briefly.

THE PRISM.

A collection of straight lines, either conical or cylindrical, representing rays of light, is called a pencil of light, and the axis of the cylinder or cone is called the axis of the pencil.

The term medium is used in optics to signify any transparent substance, that is, any substance into which a portion of the light falling upon it can pass.

The term prism in optics is used to signify a portion of any medium bounded by plane surfaces which are inclined to one another. The bounding surfaces are called the faces of

the prism; the line in which the faces intersect is called the edge of the prism; and the angle at which the faces are inclined is called the refracting angle.

The prism is to be placed so that the axis of the pencil, by which an object is seen through it, be in a plane perpendicular to the edge of the prism; and the axis of the pencil during and after its passage through the prism still remains in this plane.

One effect of a prism of denser material than the surrounding medium is to bend every ray of light passing through it, and, consequently, the whole pencil, further from the edge of the prism.

Another effect of such a prism is to decompose each single ray of white light into several rays of different colours, which rays are bent at different angles, so as to form a lengthened image of different colours, of the point from which the ray proceeds. This image is called the spectrum, and these colours the colours of the spectrum.

When, then, any object is viewed through a prism, the two following effects

arc produced. S

Istly. The ap

parent position of

the object is changed, so that,

if the prism be held with its edge

K

M

L

downwards, as in the accompanying figure, the object appears lower than it really is, while, if the prism were held with its edge upwards, the object would appear in a position higher than its actual position. 2ndly. The boundaries of the object are indistinctly defined, and fringed with colours.

Our figure represents the section of the prism made by the plane of incidence, that is, by the plane which is perpendicular to the edge of the prism, and contains the incident ray of light PQ, forming the axis of the pencil under consideration, which proceeds from one point of an object P. AQ and AR are sections of the faces of the prism; A is a point in its edge; and the angle QAR is its refracting angle. Now the ray of light, PQ, proceeding from the object at P through the medium of the atmosphere, is bent, upon entering the denser medium of the prism, from the direction Q T into the direction QR, nearer to LQ K, the perpendicular, at the point of incidence q, to the face AQ of the prism; and, upon emerging