DRAWING APPLIANCES. 41 



immediately under the top lens of the eye-piece, and clamped 

 there by means of a binding screw, shown in the figure (bottom, 

 left hand). When not in use, the prisms in their sheath are 

 swung back, as shown, so as to be out of the way of the eye 

 in observation. If now we have in proper focus a structure 

 which we wish to draw, the prism sheath is swung forward into 

 place immediately over the eye-piece, and on looking through it 

 in the direction of the eye-piece, we see once more, through 

 camera and eye-piece, the figure of the object in the field of view 

 of the microscope. The reflecting mirror upon its horizontal 

 arm should extend out to the right-hand side of the microscope, 

 and under it should be placed (for drawing) a horizontal drawing- 

 desk, the surface of which should be about ten inches below the 

 level of the eye-piece. Lay a sheet of drawing-paper upon this 

 desk, and rest the point of a lead pencil against it. If the point 

 of the pencil is under the mirror, it should now be visible in the 

 field of view of the microscope at the same time w r ith the figure of 

 the object. The point of the pencil is, however, made visible by 

 double reflection, the first time from the large mirror, the second 

 time from the silvered surface of a small prism in the point of sight 

 of the eye-piece (compare the figure) ; while the microscopic figure 

 comes directly to the eye through a small gap in the silvering of 

 this prism. If the surface of the drawing-desk does not lie at 

 the distinct visual distance of the observer, the point of the pencil 

 will be seen indistinctly, and the drawing-desk must be raised, 

 or, though seldom, be made lower. We can test the necessary 

 height by means of books laid one upon the other. The micro- 

 scopic image and the pencil are only well visible on the drawing 

 surface when a definite relation of brightness exists between the 

 two. The dimming of the paper surface is effected by a cap, to 

 be placed over the camera-prism, and which in the figure is seen 

 lying by the apparatus. Round the rim of this cap are seven 

 apertures, one open, the others closed with disks of neutral tint 

 glass of various degrees of depth, through one or other of which 

 the pencil of light coming from the mirror can be passed. Dim- 

 ming of the image itself can be effected by the diaphragm, or, in 

 some forms of the instrument, by a wheel of similar disks of 

 glass which can be rotated in the camera-setting itself, just under 

 the prisms. If the arrangement is perfect, you can draw with 

 the lead pencil the outline of the object as if drawing it in the 

 field of view of the microscope. 



