ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



277 



extent out of their position in the spectrum, is always greater with 

 increasing ordinal numbers. If the thickness, for which the dark 

 interference-band between crossed prisms corresponds with G 

 Fraunhofer, is 



= 2 (71 - 1) 



2 (-ya - «g) 



(3) 



but the thickness, for which the interference-band corresponds with 

 B Fraunhofer, is 



d' = 2 (n - 1) 



2 (^B - '^b) ' 

 then according to (3) and (4) 



d' -d = 2(n-l)\ — — *^ r , 



L2(Tb-«b) 2(7^-«g)J 



(i) 



(5) 



which shows that when the ordinal number n of the dark interference- 

 bands ascends from unity to unity, the increase in thickness, which is 

 necessary to move the band once from G to B through the spectrum, 

 forms an ascending arithmetical progression. 



The instrument can be used as a spectromicroscope alone, without 

 the polarizing apparatus, or it can be employed as an ordinary 

 Microscope if the spectral apparatus be also removed. 



Verick's Travelling or Pocket Microscope. — In this instrument 

 (figs. 46 and 47) portability is obtained, not only by the usual expe- 



FiG. 46. 



dients of reversing the body-tube in its sheath and setting the stage 

 at right angles, but also by making the two legs of the base close 



