130 THE AMERICAN MONTHLY [ApH, 



Such an illumin.ition was felt to be a desideratum in 

 quite early microscopical days, and in all the older text- 

 books will l)e found descriptions of api)aratus to serve 

 this end, rani^inj^ from sim{)le contrivances like waxed 

 paper, ground <^lass and plaster-of-Paris mirrors to lig^lit 

 modifiers, reflector screens, white-cloud condensers, 

 double parabolic specula, and many more elaborate de- 

 vices. It is pretty obvious, therefore, that nothing- new or 

 striking is likelv to be invented for the ])urpose now, 

 when the tendency is to diminish I'ather than multi{»lv 

 apparatus. 



The idea is to intensify the light and then spread it over 

 a large surface. For the intensilication I use the lower, 

 crossed lens oi the Abbe condenser, (hg. 2, b) but any 

 suitable fairly large lens of about one inch focus will do as 

 well, either a double convex orthc field lens of an eye-piece. 

 This is screwed into the lower end of a piece of tube 

 titting the sul)-stage, or under stage ring, which tul)e 

 should be a little longer than the focal length of the lens 

 employed. Just l)elow the upper end of the tube is a split 

 ring serving as a ledge, and on this, in the focal plane of 

 the dens, rests a circle of thinnish glass lightly ground 

 on one surface. The light from the fiat of the lamp is 

 condensed bv the bulTs-eve on the mirror, thrown up 

 throug-h the lens and focussed on the gi-ound glass, (.tig. 2, aj 

 which is racked or ])ushed up until almost in contact \yith 

 the slide. The image of the tlame being l)i-(»ken u]i at 

 every possii)le angle l)y the ground glass, with a little 

 maniinilatit)n one can till any sized field with a most 

 pleasant soft light, which can be employed for a long time 

 without detriment to vision. It was long ago discovered, 

 that freshly-ground glasvs ])()ssessed a peculiar property of 

 soft brilliancy which the commercial ])roduct ditl not, and 

 I get circles of the recjuired size fi^oin the glass-cutter and 

 grind them myself with a little fine emery and water ou 

 anotlier ])iece of glass until just sufficiently abraded to 

 stop any direct pencils. Besides the ordinary white glass 

 it is a great advantage to get some circles cut from dif- 

 ferent tints of blue or smoked glass, and either grind these 



