192 OPTICAL PROJECTION 



distinguish on the screen when it is there. Opera-glasses are 

 thus used in German biological classes. Lastly, for important 

 demonstrations in colleges there is the arc-light, which not 

 only gives vastly increased illumination, hut from a smaller 

 radiant, of which a larger proportion can he condensed upon 

 a small object, and from which a sharper image can be pro- 

 duced. The difference is exactly that which the ordinary 

 microscopist experiences when he changes the flat for the 

 edge of his flame. Hence the electric light gives not only 

 a brilliancy, but a sharpness, which cannot be obtained with 

 a powerful lime-light, and which is a marvellous tribute to 

 the perfection with which modern lenses are worked. 



96. Objectives. After the projecting microscope here 

 described was constructed, there remained the difficulty of 

 finding or constructing satisfactory objectives for it. The 

 greater part of the lenses which performed well upon the com- 

 pound instrument, broke down utterly upon the screen, partly 

 for the reason that when used in this way, direct or with 

 amplifiers, a field three times the usual diameter is employed, 

 while that field is not so flat as with an eye-piece. A lens of 

 -^ focus will ' project ' the whole of a blow-fly's proboscis ; 

 but every microscopist knows how little of this can be seen with 

 the same power on the usual instrument. For ordinary screen 

 work it is often important to cover a large field, if only for 

 ' finding ' reasons. I am afraid to say how many lenses passed 

 through my hands in the course of my search, kindly lent me 

 by friends from all quarters ; and the curious thing was that 

 price was found no criterion of performance. For moderate 

 powers, an excellent -j% in Seibert's series of lenses was at 

 length found, but powers of H inch and -$ inch had to be 

 worked out upon the screen itself. For large objects to be 

 shown with a focus of about 2J inches (this microscope ex- 

 hibiting up to 1^ inch diameter perfectly well) we got the best 

 results from an old photographic ' postage-stamp ' lens lent 

 nie by my friend Mr. Washington Teasdale ; and with a little 



