1G 



Note. 



microscopic comparer * in which a prism chamber (see fig, 3) con- 

 sisted of a horizontal tube with two vertical arms. These arms 

 fitted into the tubes of two Microscopes, and the horizontal 

 chamber contained four prisms. Two images, conveniently 

 apposed for comparison, appeared in the eye-piece chamber. This 

 idea was somewhat improved upon by van Heurck.f 



Fig. 4. 



It will be noticed that Dr. Ewell's design, which has been 

 independently evolved, is an advance in several respects. It 

 requires only one Microscope ; it secures par-focality of objectives, 

 and, owing to the use of only two prisms, the loss of light at_A,-A 

 (fig. 4) is avoided. 



See this Journal, 1886, p. 507. 



Op. cit., 1887, p. 4G3. 



