112 



Oji Nobert's Test-Plate and the Stri^ of Diatoms. 

 By W. S. SuLLivANT and T. G. Wormley. 



(From the' American Journal of Science and Arts' for January, 18G1.) 



The limit of the resolvability of lines, or how small a 

 space can exist between lines and still admit of their being 

 separated under the microscope, appears to be an undecided 

 point. Professor Qucckett (' Treatise on the Microscope/ 3d 

 ed., p. 238, 1855) asserts that "no achromatic has yet been 

 made capable of separating lines closer together than the 

 -— L-,th of an inch.'' In the same work, p. 245, it is 

 stated that Mr. Ross found it impossible to ascertain the 

 position of a line nearer than the ^rrnnnrth of an inch. We 

 find also on p. 512, that Mr. De la Rue, in his extended 

 examination of Nobert's test-plates, was unable to resolve 

 any lines closer than the v, ,' „oth of an inch. In Professor 

 Carpenter's work (' The Microscope,' 2d ed., p. 189, 1859) 

 this sentence occurs : " The well-defined lines on Nobert's 

 test-plates have not yet been resolved Avhcn they have ap- 

 proximated more closely than the -jf-g-i-orrth of an inch.'' 



From the foregoing it appears that actual experiment fixes 

 the limit of resolvability at about ^^^i-j^th of an inch : 

 this does not, as is said, vary widely from tlie deductions of 

 Fraunhofer and othei's, based on the physical properties of 

 light. In this connection the remark (op. cit., p. 47) of 

 Professor Carpenter may be cited, "there is good reason to 

 believe that the limit of perfection (in the objective) has now 

 been nearly reached, since everything which seems theoreti- 

 cally possible has been actually accomplished." 



On the other hand there are authorities who assert that 

 lines much closer than the .. ^ I ^ .. ^h of an inch are resolva- 

 ble. A few years since Messrs. Harrison and Sollitt pub- 

 lished ('Microscopical Journal,' vol. ii, p. 61, 1854) their 

 measurements of the strire of several diatoms, assigning to 

 Amphipleura pellucida striae as close as the -r^i-cnnrth to 

 1 3 o'ouu th of an inch. These measurements have recently 

 been repeated, and with exactly the same results, by Mr. 

 Sollitt alone ('Mic. Jour.,' viii, p. 51, 1859), who furthermore 

 expresses the opinion that striae as close as the ^ . .'„ ^ „ th of 

 an inch can, with proper means, be seen. Mr. SoUitt's 

 measurements have been adopted in the ' Micrographic Dic- 

 tionary' (1860) and most of the modern works on the 

 Microscope — no one, Professor Carpenter (op. cit., p. 188) 

 excepted, suggesting a doubt as to their accuracy ; on the 



