INVERTEBRATAj CRYPTOGAMIA, MICROSCOPY, ETC. 891 



is not now a proper test object for " high-power " objectives, and this 

 view appears to be founded on the fact that whereas at least a 

 ^ objective was formerly required to resolve this diatom, it can now 

 be accomplished by a 7[ or ^ inch. 



In the first place, there is an error in the assumption that resolu- 

 tion is essentially dependent upon the power of the objective instead 

 of upon its aperture. A ^ objective, if of only 1 • 1 numerical aper- 

 ture, will not resolve so many lines to the inch as a ^ of 1 ' 15 

 of equal quality. 



In addition to this, it is of course a mistake to consider that the 

 test depends upon the mere fact of the resolution of the markings 

 upon the object. For such a purpose, it is agreed that no one would 

 now think of using it. The real test, however, is the manner in ivhich 

 the image is shown, and by the qualiti/ of the image of a known object 

 the performance of objectives can be most readily determined by 

 practised observers. 



Fasoldt's Test Plate.* — Mr. Fasoldt has, it is said, made a test 

 plate of forty-one bands with a new machine constructed by him for the 

 execution of fine ruling, and capable of dividing an inch into 10,000,000 

 parts. The first band is ruled with lines at the rate of 5000 to the 

 inch, and the last at the rate of 1,000,000 to the inch. After the 

 million band, there are three " test bands " ruled in 50,000 lines to 

 the inch, but the lines cut of the same breadth and depth as those of 

 the quarter million, half million, and one million bands respectively. 



We have not yet seen any description of the plate, our information 

 being taken from an article, " An Evening with Fasoldt's 1,000,000 

 Test Plate," in which the writer is rapturous over the " genius who 

 dared not only to project, but to execute, a test so many years in 

 advance of microscopical science." 



Gunther's Photographs of Pleurosigma angulatum.f — After the 

 researches of Professor Abbe on the Theory of Microscopic Vision had 

 placed it beyond doubt (writes Dr. Kaiser) that " the image of fine 

 structures is not produced dioptrically, but by the interference of 

 diffracted rays," there could no longer be any question that " the 

 interference images arising from the action of diffraction do not neces- 

 sarily represent the nature of the corresponding object," and therefore 

 all attempts to determine the structure of the more difficult objects 

 (as e. g. diatom valves) by simple inspection of tlieir microscopical 

 images, must be considered a priori as utterly futile. Rows of 

 dcprt!Ssions will produce precisely the same images as actual stria;, 

 whilst on tlie other hand, by striie of different densities, may bo 

 produced the same interference-images as with an actual grating. 



In order, tluireforc, to projierly ascertain the structure of iinely- 

 organized objects, and especially to determine the structure of the 

 diatom valves used as test objects, recourse has repeatidly been liad 

 to microphotograpliy, which has proved an excellent auxiliary in this 

 department of micrographic research, and in addition has to some 



* ' Am. Journ. iMin.,' v. (ISSO) p. ICO. 

 t ' Uot. CVntmlbl.,' i. (18S0) p. GS3. 



