CORRESPONDENCE. 123 



It is the custom of most of our English makers to stop the closing 

 of the lenses at a point where the definition becomes useless. Whether 

 Mr. Tolles imagines I had predetermined that my " predictions " 

 should be verified or not, is but of little consequence as affecting the 

 facts. I really expected to find some reason for these alleged ultra- 

 theoretical rays, and that I should have to account for such an 

 appearance. Mr. Tolles, in admitting that he closes the lenses within 

 the position of proper definition, gives us the key to his fallacy. I 

 submit that my trial was correct ; we are merely dealing with visual 

 angles. Whatever these are to begin with, the simple question is 

 next, What is the loss in water and balsam ? Any after-alteration in 

 the adjusting collar will falsify the results. 



Finally, as this trial has not supported Mr. Tolles' views, he 

 seems to imply that it has arisen from " warped vision and judgment." 

 A very convenient dismissal ! If I am honoured by being considered 

 a judge in the matter, I may regret that the evidence of measured 

 aperture had not gone Mr. Tolles' way, and so left me to account for 

 the discrepancy. If the glass in question has not yet been returned, 

 Mr. Tolles may move for a new trial, and get some of his own friends 

 to be present. Though he excludes me from the list, surely he must 

 allow that there are some Englishmen that will do him justice. I 

 have no desire to be present, or to interfere further ; but to facilitate 

 the matter, I am willing to provide any apparatus that may be called for. 



Mr. Tolles proposes to have further "attested measurements" 

 made in America. I only hope that they will explain their method 

 in detail, as plainly as I have done. Let us have their names by all 

 means, and if there is one amongst them known to be capable of 

 discussing the question on the admitted laws of optical science I will 

 be happy to exchange notions with him. 



I noted the facts under the conditions named as I found them, and 

 should have recorded any measurements, however adverse to theory, 

 and sought reasons for the discrepancy subsequently. It was Mr. 

 Tolles' own desire, and not mine, that I should make the trial. I ask 

 whether his request was a fair one if he had predetermined to impugn 

 my competence to conduct the experiment properly if the result did 

 not confirm his own views? I could not accept the measurements 

 at the closest position of the lenses, for then all definition had gone. 



Yours very truly, 



F. H. Wenham. 



The Structure of Eupodiscus Argus. 



To the Editor of the 'Monthly Microscopical Jvimud.' 



Ashdown Cottage, Forest Kow, Sussex, Feb. 7, 1873. 

 Sir, — I am much obliged by a sight of Mr. Wells' letter. My 

 remarks on Eupodiscus Argus were by no means intended as exhaustive. 

 I wished to show that the drawings hitherto published were founded 

 on erroneous views, and that the real structure conformed to the ordi- 

 nary diatom type, though no doubt with variations. 



I find no difficulty in showing the aspects I figured on parts of 



