CAMERA LTJCIDA. 



101 



from the inclined sur- Fig. 54. 



faces at which they 

 join. But the ray 

 a' b' which comes 

 from the tracing-point 

 on entering the rhom- 

 boidal prism, is re- 

 flected from its in- 

 clined side b d to its 

 inclined side a c, and 

 thence it is again re- 

 flected to b in coin- 

 cidence with the ray 

 which has directly 

 proceeded from the 

 object.— A prism of 

 a different shape, 

 but constructed on 

 the same principle, 

 has been devised by 

 MM. Nachet for use 

 with a Microscope in 

 the oblique position, 



which is the one most comfortable to the delineator (see " Quart. 

 Journ.of Micros. Science," 

 Vol. viii. p. 158). 



72. It is so extremely 

 useful to the Microscopist 

 to be able to take out- 

 lines with one or other of 

 these instruments, that 

 every one would do well 

 to practise the art. Al- 

 though some persons at 

 once acquire the power of 

 seeing the image and the 

 tracing-point with equal 

 distinctness, the case is 

 more frequently other- 

 wise ; and hence no one 

 should allow himself to 

 be baffled by the failure 

 of his first attempt. It 

 will sometimes happen, 

 especially when the Prism 

 is employed, that the want 

 of power to see the pencil 

 is due to the faulty position of the Eye, too large a part of it being 



Fig. 55. 



