98 THE MICROSCOPE AND ITS REVELATIONS. 



same effect is produced by a contrivance which has been devised by MM. 

 Nachet for use with vertical Microscopes, and is much employed on the 

 Continent. It consists of a prism of a nearly rhomboidal form (Fig. 70), 

 which is placed with one of its inclined sides A c, over the eye-piece of 

 the Microscope; to this side is cemented an oblique segment E, of a small 

 glass cylinder, which presents to the ray a #, proceeding directly upwards 

 from the object, a surface at right angles to it; so that this ray passes into 

 the small cylinder E, and out from the side A B, of the larger prism, with- 

 out sustaining any refraction, and with very little loss by reflection from 

 the inclined surfaces at which they join. But the ray a' &', which comes 

 from the tracing point on a paper at the end of the base of the Micro- 

 scope, entering the rhomboidal prism, is reflected from its inclined side 

 B D, to its inclined side A c, and thence it is again reflected to #, in coin- 

 cidence with the ray which has directly proceeded from the object. As 

 the ray a' b f is necessarily oblique, the picture visually projected on the 

 paper will be distorted, unless the right side of the drawing-board be 

 raised, so that its plane shall be at right angles to a' 1)'. Of the nume- 

 rous contrivances for drawing from the Microscope, the simplest and by 

 no means the least effective, is the Neutral Tint Reflector, recommended 

 by Dr. Beale, which consists of a piece of neutral-tint glass, set in a cap 

 fitted on the Eye-piece, with which it makes an angle of 45. The 

 Microscope being arranged as in Fig. 68, the eye, looking downwards, 

 receives at the same time the image-forming rays from the eye-piece, 

 which come to it by reflection from the surface of the glass, and those 

 from the paper, tracing-point, or rule, which pass to it through the glass. 

 A simple and inexpensive substitute for this, which its inventor (Mr. T. 

 B. Jennings, U.S.) has found very efficient, maybe made by taking a 

 flat cork about 1 inch in diameter, cutting a hole in it sufficiently large 

 to enable it to fit tightly on the Eye-piece (without its cap), and then 

 making a transverse slit beneath the hole, into which is to be inserted a 

 thin-glass cover at an angle of 45. 



95. With one or other of the foregoing contrivances, every one may 

 learn to draw an outline of the Microscopic image; and it is extremely 

 desirable for the sake of accuracy, that every representation of an object 

 should be based on such a .delineation. Some persons will use one 

 instrument most readily, some another; the fact being that there is a 

 sort of a " knack " in the use of each, which is commonly acquired by 

 practice alone, so that a person accustomed to the use of any one of them 

 does not at first work well with another. Although some persons at once 

 acquire the power of seeing the image and the tracing-point with equal 

 distinctness, the case is more frequently otherwise; and hence no once 

 should allow himself to be baffled by the failure of his first attempt. It 

 will sometimes happen, especially when the Wollaston prism is employed, 

 that the want of power to see the pencil is due to the faulty position of 

 the eye, too large a part of it being over the prism itself. When once a 

 good position has been obtained, the eye should be held there as steadily 

 as possible, until the tracing shall have been completed. It is essential 

 to keep in view that the proportion between the size of the tracing and 

 that of the object is affected by the distance of the eye from the paper;, 

 and hence that if the Microscope be placed upon a support of different 

 height, or the Eye-piece be elevated or depressed by a slight inclination 

 given to the body, the scale will be altered. This it is, of ^course, 

 peculiarly important to bear in mind, when a series of tracings is being 

 made of any set of objects which it is intended to delineate on a uniform 



