1897] MICROSCOPICAL JOURNAL. 257 



A brass cap is made to fit loosely over the eye-piece of 

 the microscope so that it can be moved around and the 

 camera pointed to any point of the compass. This is 

 important as will be shown further on. Upon this is 

 placed a prism of 30°. This can be obtained at any ordi- 

 nary opticians. The prism is of ordinary crown glass 

 and is rather large as purchased but out of it two or four 

 prisms can be cut. I find it can be cut with a red hot 

 poker placed upon it along the line which it is desired to 

 cut. The cut surfaces can then be ground down with an 

 ordinary hone with emery and water. This takes some 

 time but is not essential. The microscope is placed in a 

 slanting position which is advantageous, for the camera 

 lucida can be placed upon the instrument without having 

 to turn it over until it points transversely. 



The object is viewed in the ordinary manner. Now 

 when viewed through the camera lucida, the object seems 

 to be moved towards the smallest side of the prism. 

 That is to say the ray does not go through the instrument 

 in a straight line but is bent toward the thin edge of the 

 prism and in this way it seems to move the object out of 

 the microscope to one side. When the left eye is used 

 on the microscope the thick side of the prism is on the 

 same side, i. e. the left. The object seems to be moved 

 towards the right. It is there thrown down on a paper 

 which is used to delineate it by means of a pencil. This 

 pencil is seen by the right eye and in consequence of the 

 two eyes being in use the object seen by left eye is trans- 

 parent to the paper and seems to be where the pencil is. 

 Of course such a camera lucida is not perfect. But it 

 comes into play very often. And this was the shape I 

 made it into. 



I propot^e to use a plano-convex lens with the convex 

 aide uppermost where the right eye is placed and this 

 will make it more certain. For if the lens is twelve or 

 fifteen inches focus it can be used to see the pencil point 



