152 COKEESPONDENCE. 



No. 2 against the new mode. It is as follows : — I may be mistaken, 

 and hope I am so ; but it appears to me that although the new mode 

 shows objects with greater brilliancy than before, yet it is gained at 

 the loss of perfect achromatism. For example, if I examine a valve of 

 the Pleurosigma Formosum, under my " new sixteenth," arranged in the 

 usual way, the beautiful " markings," which under a lower power (say 

 ^ or ^ inch) appear as fine crossed lines, are resolved into regular 

 rows of dots ; but without any particular colour. They are merely 

 dark dots, and nothing more. But on applying the " aquatic " the 

 said dots are immediately converted into brilliant little gems, resem- 

 bling rows of rubies ! making the object far prettier, doubtless ; but 

 the question is, is it right ? 



Perhaps some of your numerous readers will kindly give me their 

 opinion ? Meanwhile, I shall be happy to exhibit these effects to any- 

 one who may be coming this way, and may consider it worth his 

 while to call. 



Many years ago, a microscopical friend said to me, " The best test 

 for perfect achromatism in a low power (^ or ^ inch) is the minute 

 pops (or ' glands ' as they are called) seen between the lines in a 

 longitudinal slice of coniferous wood ; common deal for example." 

 And, truly, I never yet saw a ^ inch that would show them without 

 colour. 



But here is the question. Is the said colour real ? or is it caused 

 by want of perfect achromatism in the object-glass ? or is it the result 

 of some kind of dichroism, or semi-polarization ? 

 I should really be glad to know. 



I will merely add that the P. Formosum above-mentioned is pre- 

 pared in Canada balsam, in the usual way. 



Yours respectfully, 



H. U. Janson. 



The Apebture Question. 



To the Editor of the ' Monthly Microscopical Journal^ 



Padnal Hall, Chadwell Heath, Essex, Aug. 1, 1873. 



Sir, — Mr. Tolles will, I hope, excuse me from making any further 

 references to past sentences, which can scarcely affect a position that 

 must now be well understood. Whether in his measurements he did 

 close the lenses to a degree or so beyond what I had done, cannot 

 alter the principle. I have therefore finally to thank him for the 

 equanimity and good humour that he has maintained in the con- 

 troversy. 



As a word in answer to Col. Woodward's omitted letter (dated 

 May 19), I may state, that I had not the slightest wish to disparage 

 the glass sent here for trial ; for it has always been my desire to 

 avoid the publication of comparisons either against or for any exist- 

 ing makers, by which their respective partisans too often appear as 

 advertising mediums. 



