ZOOLOaY AND BOTANY, MICROSCOPY, ETC. 



949 



A dazzling white is alone seen on a black ground, witliout the 

 slightest trace of the colours of the secondary spectrum. 



I consider that this is the method to be followed to solve the 

 problem of absolute achromatism of objectives both for the telescope 

 and the Microscope." 



Eefractive Indices of Optical Glass.* — A table of refractive 



indices has recently been published under the heading of " Fraun- 

 hofer's Table of Eefractive Indices," in which the following are the 

 figures given for crown and flint glass for the lines C and G : — 



Crown : — 



Chance's soft 1'5119 



hard .. .. .. .. 1-5146 



No. 13 



No, 9 1-5268 



M. 



Flint:— 



Chance's light 



No. 3 



Chance's dense 



No. 30 



No. 23 



No. 13 .. 



These figures, however, require considerable supplementing, as in 

 the present day glass is used for optical purposes, notably by Zeiss 

 for microscope objectives, the index of which far exceeds anything 

 given in the above table, being as high as 1-8017 for flint "j- for the 

 D line. The flint is not perfectly white but slightly yellowish, which 

 is not, however, perceptible in the small pieces used for the lenses of 

 microscope objectives. We believe also that Messrs. Chance make a 

 double-extra-dense flint of 1 • 71-1 • 72 index. 



Fine Rulings. — We noted in vol. iii. (1880) p. 891 and ante p. 544, 

 the alleged issue of rulings on glass of an almost fabulous degree of 

 fineness. Mr. Fasoldt now writes | as follows : — 



" There seems to be some misapprehension in regard to my claims 

 for fine ruling up to 10,000,000 lines per inch. The truth is, I have 

 never claimed to rule that figure. I believe I once communicated 

 the fact that my ruling-engine had an index capacity of 10,000,000, 

 but it was necessary to arrange it thus in order to facilitate the sub- 

 division in the lower figures. 



" I have ruled plates up to 1,000,000 lines to the inch, one of 

 which was purchased by the U. S. Government at Washington. 

 These plates show lines truly and fairly ruled, as far as lenses are 

 able to resolve, and above this point the spectral appearance of the 

 bands in regular succeeding colours (when examined as an opaque 

 object) shows, beyond doubt, that each band contains fairly ruled 

 lines up to the 1,000,000 band. 



" Dr. Ward seems to think that the simple setting of the register 



* Engl. Mech., xxxiv. (1881) p. 56. t As low as 1-5017 for crown. 



t Amer. Journ. Micr., vi. (1881) p. 163. 



