292 COKRESPONDENCE. 



very fine platinum wire, and finally with the blow-pipe run the whole 

 together with silver solder and borax. The solder insinuated itself 

 between the coils, filled the saw kerf and ran round the diamond, 

 iiniting the whole as a solid mass. This made a very firm and sub- 

 stantial job of it. Of course a very clumsy piece of diamond will 

 serve as the turning tool or rather rubber. I assure you that this 

 is a very easy operation. 



" I hope that this contains a hint that may be of service to you. 

 You can make any use of it that you think proper. Publish it if you 

 like, for these little dodges arQ too often kept as trade secrets, to the 

 detriment of their utility. 



" Yours sincerely, 

 " H. J. Slack, Esq." " F. H. Wenham. 



Mr. Tolles and Mr. Bicknell. 



To tlie Editor of the ' Monthly Microscopical Journal.^ 



November 6, 1871. 



Dear Sib, — Enough has been said to convince minds conversant 

 with optical science of the loss of aperture on objects in balsam or 

 fluid. Mr. Tolles, however, still disbelieving, puts a case. If he is 

 clever enough to mount a diatom in balsam, between two hemispherical 

 lenses, or enclose it in the centre of a tiny spherule of hard balsam, 

 or glim, then of course it can be illuminated from all angles, and seen 

 by the full aperture of an object-glass (the refraction of the material 

 will not influence the result, as the rays proceed in. straight lines 

 through the surface). But if I understand Mr. Tolles' not very per- 

 spicuous summary correctly, he brings this profound illustration for- 

 ward to show that the angle of apertui-e is in no way diminished, when 

 the object and front lens are both immersed in fluid. Does he really 

 expect us to believe that he can obtain all his apertm-e by the back 

 combinations alone, and that these are capable of receiving or trans- 

 mitting 170°, or even anything near the least angle of 90° that he has 

 shown ? or that any of his objectives, duly adjusted for an immersed 

 object, and thus showing a large aperture when measured in air, will 

 retain the same angle when the front is immersed in a body of water ? 

 If so, further comment is useless, as it would no longer be a scientific 

 discussion of any general interest, but merely an individual one — a 

 rather hopeless attempt to convince him, by explaining primary laws 

 of refraction, or the very a, b, c, of optics. His position is an impos- 

 sible one. In a corrected high-power object-glass, when immersed, 

 the focus does not fall in the centre of the hemisphere of the front 

 lens, but is considerably beyond it. 



In the last Journal there are some remarks by Mr. E. Bicknell. 

 At page 22G, paragraph 4, he applies the terms " deception" and " mis- 

 leading people " to Messrs. Powell and Lealand, and the same to 

 Dr. Woodward, for " knowingly " putting forth work done by a higher 

 objective as that from a xV*^- I consider that this is hardly fair, and 

 is uncourteous to the gentlemen named. A scientific microscopist 

 gives the diameters with his illustrations, states the aperture, and the 



