Limits of Microscopic Vision. By Dr. Royston-Pigott. 13 



bands running longitudinally. Taking a pair of these, the cross 

 wires of the micrometer were accurately adjusted in the centre of 

 each bright space, the result for this order of spider was (making 

 the power lOuO) with the micrometer 



10000 = 15000*^ ^*^^y °'^^^^ (^'^ ^^- !>• 



Different spiders spin much thinner webs, and seem to unite several 

 according to the tension required. Another fibre measured tt^t^tt, 

 and some are discoverable 3 jo-outh. See Plate III. 



B. — The reduction by miniature will be readily understood 

 from diagrams, shown Fig. 3. There are two ways of deciding 

 the ratio of reduction: the one by examining the size of the 

 miniature itself, the other by finding the magnifying power of the 

 ajDparatus used as a Microscope. 



For this purpose it was especially mounted on the arm of the 

 Microscope used to carry the body (exhibited to the meeting). 



In these ways it was found that : — 



Immersion - Powell and Lealaud, miniatured 36-7 times at 4^ inches. 

 — 6. Gundlach „ 50-17 „ ^ „ 



„ — R. and J. Beck „ 140 „ 7 „ 



The distance between object and spider lines in the focus of the 

 positive eye-pieces varied accidentally with the length of the objec- 

 tive mount itself. But the " Beck glass " required seven inches to 

 do it justice, and also to get the miniature sutBciently reduced. It 

 was easy to form the image at any desirable distance, but then the 

 mirror could not be used very well beyond seven inches, nor the 

 micrometer held sufficiently steady without complex arrangements. 

 The one shown is simple and adequate. 



The miniature can, it is evident, be carried to any extent ; which, 

 however, is limited to certain dimensions depending upon two im- 

 portant conditions — brilliance or darkness. A very brilliant line or 

 disk is enlarged considerably, whilst a dark fine is little changed. 



If you miniature the sun's disk by viewing an aerial imao-e of 

 it formed by a 3-inch lens (100" distant), employing a magnificent 

 tV immersion, you will get a disk reduced 1000 times theoretically ; 

 and since ^^ of an inch is the diameter of the image of the sun 

 formed by the 3-inch lens, its diameter miniatured on the stage is 

 1000 times less, or 



382000 "^ iOOOOO '^^^^^y- 



See ' Proc. Key. Soc.,' No. 146, p. 428. 



