.1886-87.] Hume's Projection Microscope. 6\ 



icus, and A. Ehrenbergii can easily be detected. Plenrosigma 

 fingulatura is perhaps the most beautiful diatom for this light. 

 It varies from a milky to a deep blue ; and I consider that 

 unless where resolution is desired, this diatom should always be 

 shown by dark-ground illumination. Pleurosigma balticum, 

 Navicula major, Pinnularia, &c., take on the yellow, orange, or 

 red rays. Amongst diatoms capable of reflecting variegated 

 colours there is Podosira maxima, which is seen to consist of 

 flat radiations (Maltese-cross fashion) ; and while the body of 

 the diatom is of greenish blue, these radiations are tinted with 

 purple, sienna, and yellow. On examining this frustule with 

 ordinary means, it appears flat and uninteresting. Heliopelta 

 is also seen to much advantage : the fine colours form a most 

 gorgeous object. 



I think it a pity that there is no published account of dia- 

 toms giving a classification with respect to their appearance 

 under this illumination ; but I feel confident that our skilled 

 diatoniists will some day be induced to take the matter up. 

 Certain it is that any one engaged in this will have matter in 

 hand for profound speculation as to the causes of these exqui- 

 site diffraction spectra, and will perceive for himself that his 

 preconceived notions of the structure and forms of many dia- 

 toms were erroneous and misleading. 



[On the table were placed three microscopes giving dark-gronnd illum- 

 hiation in the three different methods above explained — viz., by the ordin- 

 ary stand- condenser, the spot-lens, and the Abbe condenser.] 



Xll.— HUME'S PROJECTION MICROSCOPE. 



By Mr Wm. HUME. 

 {Communicated March 23, 1887.) 



I SHALL do my best to first describe the general mechanical and 

 optical arrangements of this micro-lantern, and then will, I 

 hope, be able to show you, what will be more interesting, a 

 number of micro objects thrown on the screen from it. By 

 means of this its actual performance will be made visible, and 

 you will be provided with a test for judging its capabilities. 

 Until a comparatively recent date, the demonstration of 



