Messrs. Sullivant and Wormley on Nobert's Test Plate. 207 



XXV. — On Noberfs Test Plate and the St7'icE of Diatoms. 

 By W. S. Sullivant and T. G. Wormley*. 



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 

 Quekettf asserts that " no achromatic has yet been made 

 capable of separating lines closer together than the j^^ 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 -^^-^ of an inch. We find also (p. 512) that Mr. De laRue, 

 in his extended examination of Nobert's test plates, was unable 

 to resolve any lines closer than the -^^^ of an inch. In Prof. 

 Carpenter's work J this sentence occurs : " The well-defined lines 

 on Nobert's test plates have not yet been resolved when they 

 have approximated more closely than the ^^^^ of an inch/' 



From the foregoing it appears that actual experiment fixes the 

 limit of resolvability at about ^j^ of an inch : this does not, a8 

 is said, vary widely from the deductions of Fraunhofer and 

 others, based on the physical properties of light. In this con- 

 nexion, the remark of Prof. Carpenter § maybe cited : "There is 

 good reason to believe that the limit of perfection (in the objec- 

 tive) has now been nearly reached, since everything which seems 

 theoretically possible has been actually accomplished." 



On the other hand, there are authorities who assert that lines 

 much closer than the rr^, of an inch are resolvable. A few 

 years since, Messrs. Harrison and Sollitt || published their mea- 

 surements of the strise of several diatoms, assigning to Amphi- 

 pleura pellucida strise as close as the y^^p^ to j^^-^ of an inch. 

 These measurements have recently been repeated, and with ex- 

 actly the same results, by Mr. Sollitt^ alone, who furthermore 

 expresses the opinion that striaj as close as the Yfi'Wo °^ ^^^ ^^^^ 

 can, with proper means, be seen. Mr. Sollitt's measurements 

 have been adopted in the ' Micrographic Dictionary' (1860) 

 and most of the modern works on the microscojDC, — no one, Prof. 

 Carpenter** excepted, suggesting a doubt as to their accuracy; 

 on the contrary, their correctness seems to be expressly recog- 

 nized by Dr. G. C. Wallichft- 



* From Silliman's American Journal for January 1861. 

 t Treatise on the Microscope, 3r(l ed. p. 238, 1855. 

 + The Microscope, 2n(l ed. p. 189, 185!>. § Op. cit. p. 47. 



II Microscopical Journal, vol. ii. p. 61, 1854. 



If Micr. Journ. vol. viii. p. 51, 1859. ** Op. cit. p. 188. 



tt Ann. and Mag. Nat. Hist, for February 1860. 



