200 Transactions of the Society. 



beyond the stage, any such can be inserted, or changed, in an 

 instant. For ordinary use a simple series of condensers are 

 provided, adapted to the different powers. Upon these condensers 

 taking up the whole parallel beam, bringing it down to the 

 required size, and so bringing it down at an angle approximately 

 corresponding to that of the objective, or rather somewhat within 

 it, depend both (1) the illumination and (2) the definition. A 

 good objective will not give good definition with an unsuitable con- 

 denser, nor will its field be at all evenly illuminated. 



The stage consists of a revoMng diaphragm-plate ; that is, of the 

 very plate itself. A great point is, the most absolute simplicity ; 

 and nothing can be so simple as a perfectly free and open flat 

 plate, with two ordinary stage springs to hold the slide. Any 

 slide, or trough, or object, or apparatus can be adjusted on this 

 without difficulty. Moreover, it is often desirable to have the 

 apertures close to the slide ; while the slide can always be elevated, 

 if desired, by a packing of wood or card. 



Of objectives not much need be said. For large whole insects, 

 or other large objects (which can be shown brilliantly up to 

 IJ in. diameter), I still find one of the old-fashioned double- 

 piano un-achromatic form, a little over 2 in. focus, most useful. 

 By careful adjustment of its stop in size and position, and adapta- 

 tion of the condenser, such a lens will give much better results 

 than might be supposed, and no achromatic form of the same focus 

 will cover nearly so large a field. Such work is scarcely micro- 

 scopy, but nevertheless such objects often have to be shown. With 

 achromatic powers the great difficulty was to get lenses which 

 would give a picture flat to the edges. It must be borne in mind 

 that the screen shows an object three times the diameter of the image 

 utilized from the same lens in the compound Microscope. A lens 

 may be fairly flat to the edge of the compound field ; but that will 

 be the mere centre of the screen field. A ery few lenses, out of 

 many I tested, but broke down here. Absolutely the only half- 

 inch powers that really gave a flat image, so far as my own trials 

 went, were an old formula of the late Mr. A. Koss, lent me by 

 Mr. Curties, and the 40° with correction collar of Messrs. Powell 

 and Lealand, lent me by Mr. Crisp. An old Gundlach lens 

 . lent me by Mr. Teasdale, however, enabled me to get through 

 Mr. Curties 6/10 and 4/10 lenses which performed well. The 

 lower achromatic powers had to be specially worked out on the 

 screen itself by Mr. Herbert Newton, the only good and flat lens 

 I could find out of dozens, being an 8/10 made by Zentmayer. 

 Though I think Mr. Newton's is still better, chiefly in Uachness 

 of image, this last is an extremely fine lantern lens. But there 

 are doubtless many objectives I have not tried which will give good 



