The Helmholtz Theory of the Microscope. By J. W. Gordon. 423 



Illumination of the Stage. 



It is, however, quite possible to guard against loss of resolution 

 in this way by taking care to limit the luminous field of the 

 instrument to so much only as is to be photographed. If all the 

 transmitted light fall thus within the angle of the upper objective 

 you will obtain very respectable pictures in that way. There is 

 exhibited to-night a photograph of Plcurosigma angulatum magni- 

 fied 2500 diameters which was made in this little Microscope with its 

 home-made compounding draw-tube and without any screen. It 

 is, as you will observe, much spotted with the shadows of specks 

 of dust upon the lenses, but apart from that it is a passable photo- 

 graph. 



But, although much may thus be clone with only the simplest 

 appliances, the oscillating screen does so much to improve the 

 pictures that I cannot imagine that when once its capabilities are 

 known anybody who intends to do much in the way of photo- 

 micrography will work without it. It adds very little to the appa- 

 ratus and it places this simple contrivance of the compounding 

 draw-tube theoretically on a level with the projection ocular and 

 its optical bench. In fact it is better, because stiffer than the 

 optical bench. There is no risk whatever of displacement due to 

 vibration with this appliance and you could work as easily on 

 board ship as on shore. 



By means of the screen we scatter the light of the image formed 

 by the first objective, and so enlarge the angle of the beams which 

 enter the second objective, thus filling its aperture and enabling it 

 to yield a perfect reproduction of the image formed in the first focal 

 plane. 



The photographs which you see here this evening have all 

 been produced by the simple apparatus that you see upon the 

 table. My practice is to use, as the case may be, a £ or ^ as 

 the principal objective, and by doubling this with a J-in. to 

 produce a negative with a magnification of about 200 or 400 

 diameters. Such negatives will easily stand camera enlargement 

 up to four or five times, and I therefore print in the same Micro- 

 scope, using the simple draw-tube and a 2-in. objective as the 

 photographic lens, the negative being, of course, mounted on the 

 stage. In that way anything up to 2000 diameters magnification 

 can be obtained. Of the merit of the process you will judge for 

 yourselves. My results do not compare with fine photomicro- 

 graphs, for this is in many respects a makeshift piece of apparatus. 

 In fact, I am not sufficiently well satisfied with any part of it to 

 include a detailed description in the present paper, but such a 

 description is at the service of anyone who is sufficiently interested 

 to inquire for it. Perhaps I may be allowed to say here that my 



