288 CORRESPONDENCE. 



different persons ; differences in tlie skill and eyesight of the observers, 

 differences in the modes of illumination, differences in the same 

 nominal subject of examination, differences of angular aperture, and 

 so on. These, it is plain, are more or less unavoidable; but there is one 

 source of embarrassment, which, though it has often been mentioned 

 before, can never be mentioned often enough, for evils are never 

 remedied unless a loud outcry is raised against them. I allude to the 

 total absence of any standard of magnifying power from which we 

 are now suffering. I know of ^ths which amplify much more than 

 •|-ths, of -gths which exceed -j^ths, of yVths which are equivalent to 

 -jigths. In fact, matters have now come to such a pass, that an inex- 

 perienced purchaser can sehhmi know much more about what he is 

 buying than that it is an object-glass. It would be a great boon to 

 the world of microscopists if the Royal Microscopical Society could 

 put forward a standard measure of linear dimensions for a given fociis, 

 and that our great makers would at least try to approximate to it ; 

 for the present system is an affront to common sense and common 

 honesty. 



But, notwithstanding these difficulties, comparison between English 

 and foreign objectives is going on, slowly but surely, sometimes 

 noisily, oftener silently, yet still going on, searching for facts, and 

 awaiting a final verdict. As yet our country does not appear to 

 have been worsted in the trial, although Mr. Mayall would probably 

 think that the case has already gone against her, and that foreigners 

 are of his mind.* There is one circumstance, however, that English 

 opticians would do well to kcei) continually before them, that Europe 

 has secured an immense advantage over her in the matter of price, and 

 that nothing but quality can ever make head against cheapness. 



As bearing upon this question, and with a full sense of the 

 embarrassments which siirround it, I would venture to make a few 

 remarks in connection with the interesting note from Mr. Kitton in 

 your last number. 



As far as I am able to judge, I should say that Ross and Co.'s 

 patent ^th is about a match for his Beneche's No. 7 in magnifying 

 power, though it may exceed it in angular aperture. On applying this 

 objective to the examination of the diatoms which he has named, I 

 found that with perfectly direct candle-light, mirror and diaphragm 

 being both excluded, the B eye-piece revealed the striae on P. angu- 

 latum most beautifully, and without the smallest change of the condi- 

 tions brought out the arrangement of the terminal striae perfectly well. 



With the help of lamp and condenser, and using the C eye-piece, 

 the checker-work of P. intermedium was exhibited most distinctly, and 

 the costfe of Cymhella Ehrenhergii plainly seen to be composed of 

 flattened beads. 



Pinnularia peregrina is a more difficult object than Cymhella Ehren- 

 hergii, but in one frustule the transverse lines upon the costae were 

 shown almost vividly ; while those on Nitzscliia sigmoidea stood out 

 quite distinctly for all their closeness. 



The transverse markings of Synedra rohusta were distinctly re- 

 * See 'M. M. J.,' Feb. 1869, p. 90. 



