SCIENCE- G OS SIP. 



CONDUCTED BY P. SHILL1NGTON SCALES. F.R.M.S. 



Eoyal Microscopical Society. — June 19th, 

 William Carruthers, Esq.. F.E.S., President, in the 

 chair. At the special general meeting several 

 alterations in the by-laws were put and agreed to 

 unanimously. At the ordinary meeting Mr. T. H. 

 Powell exhibited Coscnrodircus asterompltahai under 

 a new -^th-inch apochromatic oil-immersion ob- 

 jective ; Mr. J. W. Gordon read a paper entitled 

 " An Examination of the Abbe Diffraction Theory of 

 the Microscope," in which he stated that the above 

 long-accepted explanation of the phenomenaof high- 

 power microscopic observation had been adopted 

 on insufficient proof, and would not bear the test 

 of critical examination. The Abbe theory claimed 

 that pictures formed by the microscope of very 

 minute objects were due to diffraction images 

 originated by the object, and that when the oblique 

 rays of light by which these diffraction images 

 •existed were excluded no image of the object was 

 possible. This theory had been experimentally 

 illustrated by Professor Abbe by means of a 

 grating on the stage of the microscope and a 

 series of diaphragms behind the microscope object- 

 glass with slits to partially exclude oblique rays; 

 Mr. Gordon showed that, although under such 

 favourable circumstances diffraction effects were 

 produced by fine objects on the stage of the 

 microscope, these effects did not appreciably in- 

 fluence the form of the image. He also showed 

 that the experimental results produced by the 

 .above-mentioned diaphragms, which were adduced 

 to prove the theory, were due to a diffraction effect 

 produced by the diaphragms themselves, and not 

 by the grating on the stage of the microscope, 

 the same results being obtained with an aerial 

 image of a grating projected upon the stage by a 

 lens in place of the actual grating. He maintained 

 that in the microscope, as in the telescope, it was 

 necessary to eliminate diffraction effects as far as 

 possible by making lenses of larger aperture, and 

 not, as in Abbe's theory, to include as many diffrac- 

 tion phenomena as possible. Diagrams in illus- 

 tration of the paper were thrown upon the screen, 

 and the various experiments referred to were 

 exhibited under a number of microscopes. Pro- 

 fessor S. Thompson regretted that he had not 

 heard the first part of the paper, and had not had 

 time to read the advance copy of the paper which 

 had been sent to him. He entirely agreed with 

 Mr. Gordon in rejecting the explanation of the 

 Abbe theory given by Niigeli and Schwendener, 

 but found himself at variance with Mr. Gordon on 

 almost every other point, and proceeded to discuss 

 several conclusions arrived at in the paper. Mr. 

 Julius Rheinbero- having criticised the paper ad- 

 versely at considerable length, Mr. Conrad Beck 

 said he did not think it possible for anyone who had 

 followed the experiments described by the author 

 to dispute his contention that the effects observed 

 were produced by the diaphragm behind the ob- 

 jective. The proof that the effects described were 



entirely due to this was shown by the fact that if 

 any of the conditions were altered the experiments 

 did not succeed, and there was no reason why they 

 should not succeed if the Abbe theory were cor- 

 rect. Mr. Gordon contended that he was entitled 

 to the support of Professor Thompson, notwith- 

 standing the impression his speech had probably 

 left on the minds of those present. Professor 

 Thompson agreed with him in throwing over 

 Niigeli and Schwendener's explanations, but con- 

 sidered it wrong to throw over the Abbe theory : 

 whereas the quotation at the beginning of the paper 

 made it clear that Professor Abbe had himself 

 thown it over. In doing so, however, he had promised 

 to elaborate it further. As he had not yet done this, 

 one was obliged to pick it up where it might be 

 possible to find it, and so he was obliged to go to 

 Niigeli and Schwendener's book. In reply to Mr. 

 Rheinberg, Mr. Gordon said it would take too much 

 time to follow him through the many matters 

 dealt with, and confined himself to showing that 

 Mr. Rheinberg was in error on a point fundamental 

 to his remarks. 



Cleaning Desmids. — Dr. G. H. Bryan contri- 

 butes to the "Journal of Applied Microscopy" a 

 method of cleaning desmids from the dirt and 

 flocculent matter with which they are commonly 

 mixed. The requisite apparatus consists of one or 

 two shallow porcelain saucers or photographic 

 dishes, an old pomatum-pot, and a stylographic or 

 fountain pen-filler, together with the gauze strainer 

 of a coffee-pot with which to strain out large pieces 

 of dirt. The material is strained into one of the 

 porcelain or other dishes, and after an interval of 

 not more than half a minute the dish is inclined 

 to one side, a gentle rocking motion being given at 

 the same time. Any desmids will collect in a 

 bright green line or patch at the edge of the re- 

 ceding water, from which they are picked up by 

 means of the pen-tiller. By working the water 

 round the edge of the dish the desmids may be 

 drawn into green patches in almost any desired 

 part of the vessel, and one lot after another picked 

 up until there are none worth troubling about. As 

 the desmids are removed they are transferred to 

 the pomatum-pot, where a drop of Zenker's fixative 

 suffices to fix them. Zenker's fixative is made up 

 of 5 per cent, of sublimate and 5 per cent, of 

 glacial acetic acid dissolved in Midler's solution, 

 which is itself composed of two to two and a half 

 parts of potassium bichromate, one part of sodium 

 sulphate, and a hundred parts of water. A repe- 

 tition of the rocking process enables the desmids 

 to be again collected, taken up with the filler, 

 and transferred to a dish of clean water, being de- 

 posited in a patch near the edge of the dish. This 

 latter is again rocked to wash away the fixative, 

 together with any foreign matter that has survived 

 the previous process. The desmids'should be again 

 transferred to clean water to ensure the removal of 

 any trace of fixative, and if they are to be stained 

 the stain can be washed away in the same manner. 

 This method of washing involves less loss of speci- 

 mens than the ordinary decantation method, pro- 

 viding that care be taken to deposit the desmids as 

 close together as possible after each transference ; 

 whilst ihe process does not take more than from 

 half an hour to an hour, so that the fixative has 

 not time to injure the colour of the specimens. If 

 much foreign matter — e.g. peat — is mixed with the 

 original gatherings, the whole may be left in a wide- 

 mcutbed bottle in the light for a day or two, when 



