G. GENERAL NATURAL HISTORY AND ZOOLOGY. 267 



solar line D. Monthly Microscopical Journal^ December^ 



1875. 



nobert's test-plate. 



In a paper upon the measurement of the bands of Nobert's 

 test-plate, Mr. J. A. Brown, F.R.S., arrives at the conclusion 

 that visibility of lines of the same width increases as the 

 distance between them decreases ; that parallel lines are 

 least visible when there are only two, and increase in visibil- 

 ity with their number; that Nobert's test-lines fail as a test 

 for the microscope, especially in the highest bands, from the 

 incapacity of the machine to make separate lines at less in- 

 tervals and of less width tlian tgoWo" of an inch ; they also 

 fail, in all probabilit}^, on account of the faintness of the tint 

 or shade of the lines made on the retina. Proceedings Roy- 

 al Society^ No. 163. 



holler's probk-platte. 



Professor W. Morley, of Hudson, Ohio, has published an 

 excellent and exhaustive article on Measurements of Mol- 

 ler's Diatomacean Probe-platte. In a table appended, Nam- 

 cula crassinervis is given as having 78 to 87 striae in 0.001 of 

 an inch ; Nitzschia curmda^ 83 to 90 ; and Amphipleura pel- 

 lucida^^l to 95 in 0.001 of an inch. Among the causes af- 

 fecting the resolvability of a given diatom the author omits 

 one that, even in tolerably experienced hands, is of no little 

 moment the difficulty in securing precisely the same per- 

 fect illumination upon different occasions : a difficulty so 

 great that Messrs. Dallinger and Drysdale have contrived a 

 special apparatus for this purpose, and the very best attain- 

 able results, as they show, depend upon such delicate manip- 

 ulations of the source of illumination, as well as its charac- 

 ter, that it is only in the ordinary way accomplished, as one 

 might say, by accident. Monthly Microscopiccd Journal, 

 Jlay, 181 Q. 



nobert's test-plates. 



Mr. W. A. Rogers, of Cambridge, already favorably known 

 for his fine rulings on glass for micrometers, test-plates, and 

 the diffraction spectrum, has suggested a possible explana- 

 tion of Nobert's method of ruling his test-plate ; assuming 



