Testing an Ohject-glass, &g. By Q. W. Royston-Pigoii. 917 



former size : they had just been nine-tenths the globule diameter, 

 they were now one-ninth 1 



Selecting globules about the 100,000th of an inch — (easy of 

 representation by separating the spider lines) under a power of 

 1000 — by searching the illuminated field, the tiny illumination 

 could still be caught by the best glasses I possessed. The exquisite 

 truth of these reflections by condensing the light down upon the 

 objects through the object-glass, forms the most thoroughly 

 searching and infallible test of the excellence of a glass with which 

 I am acquainted. 



For some months a series of observations more and more 

 confirmed me in this opinion. 



The manipulation is somewhat delicate ; the hght or flame 

 must be placed so that its distance measured by the path of the 

 illuminating rays shall equal nearly the distance of the eye from 

 the stage. If immersion liquids are used, those globules must be 

 selected which appear brilhantly illumined on an intensely black 

 ground. I have used a tin cylinder somewhat contracted at the 

 top and perforated with a pigeon-hole aperture to project the 

 illuminating rays horizontally.* 



A transcend ently fine diff'racting ring of light of the most 

 astounding attenuation may be discovered with the finest glasses, 

 appearing much finer than the spider lines of the micrometer, the 

 thinnest of which is the 10,000th of an inch mounted by 

 Mr. Browning. Now what this represents on the stage when an 

 object-glass magnifies 1000 times to the eye, I leave my friends to 

 calculate for themselves. But many persons have agreed with mo 

 that it looked, though then enlarged a hundred times, much finer 

 than the web in question. 



Practical opticians know full well the extreme difficulty of 

 viewing mercurial globules under very high power, and brightly 

 illuminated by oblique rays. 



Tins method shows true images of such surprising distinctness 

 and incredible reduction as almost to defy adequate description. 



I had the pleasure of exhibiting these appearances to 

 Mr. Stephenson last spring, and to Mr. Curties recently. 



* Tlio polished tin entirely stops tlie radiation of heat upon the observer's head, 

 which is sometimes somewhat near the illuminator. 



