CORRESPONDENCE. 151 



December meeting of the Eoyal Microscopical Society, is considerably 

 behind the times. I exhibited a stand with the same attachment 

 before the Illinois Microscopical Society in December, 1870. It had 

 three milled screws, so that it was not necessary to use a screw-driver. 

 Dr. H. A. Johnson, of this city, has a stand that is central with ^'j-th 

 objective, and there is not a so-called concentric rotating stand of 

 English make that I have seen in this country that is central with f ." 

 Mr. Bullock has sent us an illustrated description, which certainly 

 bears out some of his ideas. 



A Concentrated Mode of Mounting. — The ' American Naturalist ' 

 states that Mr. C. H. Eobinson, of Cleveland, contributes to the 

 " Postal Micro-cabiuet Club " a slide illustrating a method of 

 mounting where the space under a single large cover-glass is occupied 

 by a considerable number of small circles with an object in each. He 

 makes the circles of white zinc varnish, and sometimes adds a circle 

 to the edge of the cover-glass as a finish. This method of mounting, 

 the appearance of which is decidedly handsome, is particularly appli- 

 cable to disjjlaying several varieties of one species (as of selected 

 diatoms or of Foraminifera) on one slide, or to presenting in contrast 

 different methods of preparing the same species. 



COKEESPONDENCE. 



On the Immersed Aperture Question. 



To the Editor of the ' Monthly Microscopical Journal.'' 



Sir, — I can by no means acquiesce in the statement made by 

 Mr. May all in the last Journal. I will make a brief remark only on 

 the first .paragraph. 



The adjustment seems to be a stumbling-block for those advo- 

 cating an extra immersion theory. We have now in use thousands 

 of serviceable immersion object-glasses capable of defining most tests, 

 and which have no adjustment, as they are set for an average thick- 

 ness of cover. They answer well, because in the immersion system 

 the errors of cover-aberrations are nearly eliminated, and with a 

 balsam intermedium they would be inappreciable. The apertures of 

 these lenses are, I presume, taken by the usual sector method. Place 

 a slide with object in focus mounted dry with fluid intermedium, and 

 measure the aperture through the glass slip. Take another with an 

 object in balsam and again measure the aperture the same way. The 

 measurements will be similar. The balsam has given no increase. It 

 was known when most of us were children that a close position of 

 the lenses gave increase of aperture, but I cannot allow that this is 

 attributable to any immersion principle. I have before given this 

 answer. 



VOL. XV. M 



