OPTICS. 291 



motion for turning round the circular piece of 

 wood B, with all the parts affixed. 



C is a brass tube covered with seal skin, which, 

 screwing into the middle of the circular piece of 

 wood, becomes a case for the uncovered brass tube 

 D, to be drawn backwards or forwards in. E, a 

 a smaller tube of about one inch in length screwed 

 to the end of the larger tube D. To the end of 

 this the microscope must be screwed. 



.5 is a convex lens, whose focus is about twelve 

 inches, designed to collect the sun's rays, and 

 throw them more strongly upon the object. G, a 

 looking-glass of an oblong figure, set in a wooden 

 frame, fastened by hinges to the circular piece of 

 wood B, and turning about with it, by means of 

 the above-mentioned catgut. 



H, a jointed wire passing through the wooden 

 frame, to enable the observer (by putting it back- 

 wards or forwards) to elevate or decline the glass, 

 according to the sun's altitude. 



The extremities of the catgut are fastened to a 

 brass pin ; by turning which it may be braced 

 up, if at any time it becomes too slack. This pin, 

 lying behind, could not be shown in the plate. 



When this microscope is employed, the room 

 must be rendered as dark as possible ; for on the 

 darkness of the room, and the brightness of the 

 sunshine, depend the sharpness and perfection of 

 the image. Then putting the looking-glass G 

 through the hole in the window-shutter, fasten the 

 square frame A to the said shutter by its two 

 screws and nuts, 1 1. 



This done, adjust the looking-glass to the ele- 

 vation and situation of the sun, by means of the 

 jointed wire H, together with the catgut pulley; 

 for the first of these raising or lowering the glass, 



u 2 



