A Top Stop for the Microscope. Bt/ J. W. Gordon. 5 



whether it is or is not possible to improve the performance of a 

 given objective by suppressing its centre, and I have therefore 

 made no attempt to work with objectives of high power. On the 

 contrary, the lens with which I have worked is an oil-immersion 

 of N. A. = 1, the narrowest angle to which oil-immersion lenses 

 are made. It has not been practicable to go to lenses of lower 

 magnifying power, because tlie scale of my original negatives is, 

 as matters stand, very inconveniently small. It unfortunately 

 happens that a railway tunnel runs within fifty yards of my street 

 door, and about once in every five minutes through the day and 

 the working night an earthquake shakes the house from attic to 

 basement. My best chance of getting a photograph at all is to cut 

 in between the earthquakes, and therefore I have to be content 

 with short exposures. Short exposures, when a top stop reduces 

 the image-forming beam to a mere edge, imply low magnification, 

 and thus it happens that all the photographs which I have to show 

 this evening are enlargements from negatives of about 300 dia- 

 meters magnification. So much for the lens employed. 



As to the illuminant, I have used a Welsbach mantle and a 

 yellow screen. The Welsbach mantle, thrown far enough out of 

 focus to destroy the image of the mesh, gives a very excellent 

 light, quite strong enough for my purpose. A pin-hole aperture 

 placed between the condenser and the lamp, and situated in the 

 conjugate focus of the condenser, cuts off superfluous light and 

 confines the illumination to the area under observation. The 

 yellow screen, employed with an orthochromatic plate, enables me 

 to photograph what I see — a consideration sometimes overlooked 

 by photomicrographers. A superstitious belief in the superior 

 virtue of blue light leads them to use ingeniously-devised blue 

 screens, with the result that if they obtain sharp images at all, 

 they so obtain images of such things as the eye hath not seen, 

 for under these conditions a human eye sees one thing, and a 

 photographic plate sees another. The residual yellow limns one 

 plane in the eye, the ultra-violet sketches a higher plane in the 

 camera, and if these two planes in the object carry different designs 

 — as commonly they do with high-power objects — the result of an 

 exposure is unintelligible to the mere photographer, and exasperat- 

 ing even to the most chastened microscopist. To spare myself 

 these vexations I use, both for focusing and photographing, a 

 strongly yellow light. 



For my purpose a plate of very fine grain is of course indis- 

 pensable, since my negatives are destined to undergo extreme 

 enlargement. I therefore use an orthochromatic process plate, and 

 as such plates are to be had which are much more rapid than 

 ordinary process or lantern plates, the highly important condition 

 which enables me to work with comparati\'ely short exposures is 

 also satisfied. 



