34 2 APPENDIX. 



the window towards which the microscope points is a stout immove- 

 able shutter, having a square opening to receive a moveable piece 

 which fits into, it with a rebate, and is held in position by four wooden 

 buttons. An aperture is cut in this moveable shutter (see fig. 414) of the 

 same diameter as the short body of the microscope, and in a direct line 

 with it ; a light-tight connection is made between the two by a sliding 

 brass tube (l>) fitted to the shutter. This aperture can be opened and 

 closed at will, to make the exposures, by a brass plate (c) playing 

 over the outer face of the shutter on a pivot, which, passing through 

 the shutter, is worked by a handle (d] from withing the room. 



" This brass plate is sunk into a shallow space cut in the shutter 

 so as not to project beyond its surface. Over the plate and covering 

 the aperture is fastened the glass cell (e) containing the blue copper 

 solution. Immediately below the edge of this cell a piece of brass 

 tubing (/) thirteen inches long, is screwed to the shutter, carrying 

 at its extremity the microscope mirror (g), accurately centred oppo- 

 site the aperture in the shutter. This mirror is adjustable from 

 within the room by means of two steel rods (/i/i) attached to its 

 framework by ball and socket joints, and projecting into the room 

 through small holes in the shutter. One of these rods moves the 

 mirror upon its vertical, the other upon its horizontal axis. The 

 heliostat stands on an iron shelf, outside the window, in such a posi- 

 tion that its mirror is a few inches only distant from the microscope 

 mirror and in a north-westerly direction from it, fig. 417, a. 



' The frame for the plate-holder, instead of standing upon a table, 

 is supported upon a narrow walnut car, running upon an iron track 

 ten feet long, laid upon the floor at right angles to the plane of the 

 window (see fig. 417). This car consists essentially of a base made of 

 four pieces of wood joined together so as to leave an opening in the 

 centre eight inches square, and two stout uprights, connected by a 

 cross piece, which rise from the side pieces of this base and have 

 a V-shaped way cut on their inner faces to receive the sliding sides 

 of the top of the car. This top can thus be adjusted to any height, 

 and clamped in position by wooden binding screws, so that negative 

 plates of different sizes may be used if desired, and centred to the 

 axis of the microscope body. The track (see fig. 415) consists of two 

 wooden rails (cc) an inch high, screwed to the floor, upon which in 

 turn are screwed flat iron rails (bti) whose inner edges project half 

 an inch beyond the wooden rails. These iron rails are cast with a 

 A-shaped projection on their upper faces, and the base of the car is 

 furnished with small brass wheels (aa) correspondingly grooved to 

 run on these projections. The car can be firmly fixed upon the 

 track at any position by the following means. Through a hole 



