ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 415 



By this means all the inconveniences attendant upon engravincf 

 the divisions at the bottom of the cell are avoided. The divisions 

 may be as exact and as strongly marked as possible, the image 



Fig. 82. 



depending entirely upon the intensity of the photograph and its size 

 on the reciprocal distance of P and 0." 



Fasoldt's Test-plate.* — Professor E. Hitchcock does not consider 

 that the diffraction spectra alleged to have been seen by Mr. Fasoldt are 

 any proof of the presence of the separate lines claimed, and " would 

 like Mr. Fasoldt to inform us how fine the individual lines of his 

 wonderful plate are ? If the plate has 1,000,000 lines to the inch, 

 the individual lines cannot be broader than half a millionth of an 

 inch. Can such fine lines be ruled ? Then it is a question in 

 mechanics, whether a tool can be made so steady that it can draw a 

 line without a tremor of half a millionth of an inch— for if not, then 

 the lines of the plate must run together. 



" In regard to the first question, there is already some evidence 

 that Mr. Fasoldt's assumption is not justified. Professor W. A. 

 Eogers ruled a plate with his machine set for 500,000 lines to the 

 inch, making every fifth and tenth line longer than the rest. He 

 then measured the long lines, where they projected from the band 

 and found that they were so broad, that they overlapped each other, 

 leaving no spaces between them. Evidently, therefore, the band of 

 500,000 lines did not consist of distinct lines. The spectra were, 

 nevertheless, clear and bright. Hence, we are forced to conclude 

 that the spectra do not prove that Mr. Fasoldt's plate contains 

 1,000,000 lines to the inch." 



We have not seen Mr. Fasoldt's claim as to the diffraction spectra 



* Amer Mou. Micr. Journ., iii. (1882; pp. 52-3. 



