DESCRIPTION OF DR. CORING's ENGISCOPK. 173 



and the same focus operating - by lamp-light, with another 

 isosceles convex of same power and diameter screwed to it : 

 these curves have been found by experiment the best both 

 for refracting the maximum of light, and for preventing 

 any loss of it by reflection from the convex surfaces of 

 the lenses, 'h is the cap of one of the eye -pieces in dotted 

 lines ; it is supposed to be removed for the purpose of 

 applying the camera lutida, fig. 1 1, which is thrust on over 

 the eye-tube, a side view whereof is represented at fig. 12. 

 By far the best species of camera lucida is, I think, this 

 (which is the invention of Professor Amici), and is com- 

 posed of a simple piece of thick plate glass, with truly 

 parallel surfaces, placed at an angle of 45°, and reflecting 

 an image while we see through it, for it allows the point of 

 the pencil, the drawing on the paper, and the image, to be 

 seen at the same identical moment, without any dodging 

 or effort. Care must be taken to have a sufficient thick- 

 ness of glass, to prevent the visual pencil from reach- 

 ing the inferior surface, otherwise there will be two 

 images. 



Fig. 13 is a micrometer, to be placed in the field-bar 

 of one of the lowest eye-pieces. It is made of glass, with 

 divisions of 40 or 50 parts to the inch, which are filled 

 with black lead, well burnished in, and secured by 

 another piece of glass being cemented over them. 



Figure 14 is a square bar, sliding up and down in the 

 square tube in the stage, k, fig. 9. It carries the illumi- 

 nator,/, when it is thought advisable not to load the body 

 with it. 



