202 Transactions of the Society. 



cornea, a 1/5 amplified, at 25 feet distance, passes enongh light to 

 exhibit it 2500 diameters. For this I prefer a Kellner eye-piece as 

 condenser. What can be done with diatoms I do not know, having 

 had no opportunity as yet of trying the completed instrument with 

 those excellent condensers made for this express purpose ; if any one 

 interested is disposed to assist me in such trials, I shall feel exceed- 

 ingly obliged, this class of work being rather out of my range, and 

 depending so much upon experience, and technical knowledge, and 

 apparatus, which I do not possess. So far as definition of the 

 objectives goes, I have ascertained that a Zeiss homogeneous 1/8 

 will give every dot in an image of N. hjra 4"feet long ; but my 

 condensers for lower powers do not give sufficient illumination. 

 The best I have myself done with high powers has been, with this 

 same lens, to show the cyclosis in VaUisneria so that the motion of 

 the chlorophyll granules could be clearly seen about 12 feet away 

 from the screen ; but I am satisfied that more than this is possible 

 with proper arrangements, and probably one of Powell and 

 Lealand's new formula immersion lenses would do better than the 

 Zeiss, fine lens though the latter is : for work in the dark, water is 

 certainly preferable to oil or chemical solutions. Now that my 

 instrument is done with for " pattern " purposes, I hope to experi- 

 ment further in this direction. 



As to shdes, the bettir the slide the more can be shown in it, 

 of course ; but no more is required in the vast majority than any 

 microscopist would select for himself with a pocket lens as the 

 " best " out of half-a-dozen. In special cases, whatever he wants 

 to exhibit, he must of course see is really clear in the slide. The 

 conjugate focus on the screen is so enormously long in proportion 

 to the other, that where no amplifier is employed "depth of focus" 

 is of course diminished. Hence biological or other sections should 

 be chosen rather thin, that the image may be as nearly in one 

 plane as possible. This is the chief point to keep in mind ; but I 

 have found no diflficulty amongst any lot of sections, in selecting 

 one which would answer all purposes. 



Before concluding, I should wish to suggest two ways in which 

 a good lantern Microscope may, I think, render service to general 

 microscopic work, besides its more proper task of screen demon- 

 stration. Original research is of course out of the question, since 

 its magnifying power must always be far beneath that of the com- 

 pound instrument, simply because an illumination amply sufficient 

 when gathered into the small area of the pupil of the eye, is so 

 tremendously diluted when spread over a large screen. Moreover, 

 quite coarse detail on a screen cannot be seen at any distance : an 

 image of a diatom a foot in diameter is really veiy coarse, and 

 quite brilliant ; but it is surprising at how small a distance ofi" the 

 details cease to be distinguished by the unaided eye, while still 



