220 



Dr Royston-Pigott, On a new method of [Dec. 2, 



A spider-line micrometer being inverted below the stage and 

 the lines brought into the field of view, a glass stage-ruled-miero- 

 meter was placed so as to intercept the miniature of the webs, the 

 head of the micrometer being divided into one hundred parts, and 

 each revolution representing the hundredth of an inch, the distance 

 of the spider-lines from their miniature image was then carefully 

 measured, and for convenience was limited by a tube strongly 

 built up of screwed "adapters." In the case of Messrs Beck's 

 excellent ^V immersion the tube was increased to seven inches, 

 ten inches being the standard distance for the best performance of 

 the glass. But although it is clear optically, that the miniature 

 will be precisely as much reduced as the objective magnifies in 

 a reversed position of object and image, yet for certainty the 

 micrometer armed as before with the objective was used and tested 

 as a microscope on the same ruled micrometer with the same 

 result. See Fio. 5. 



At ten inches the -^lyth Beck immersion magnifies 200 times, 

 and at seven, 140 times. I insert here a table of some measure- 

 ments actually made of the reduction in miniature. 



No. 



(1) 



(2) 



(3) 



Distance of 

 object from 

 miniature. 



6|^ iiiclies 



63 



(4) 



(5) 



(6) 



(7) 



(8) 



H „ 



H „ 



H „ 



Focal length of 

 objective. 



Koss 1 inch 



Eeduction in 

 miniature. 



6 '07 times 



Diameter of 



TTrnTTTrth Web. 



.th 



Ross 1^ „ 



Wrayi „ 



Wray i „ 



A.. Ross ^ „ 



Powell and 



Lealand 

 i immersion 



Ditto Ditto 

 1875 



Beck 



~j immersion 



1878 



2-85 



13-40 „ 



29-50 



27-60 „ 



58-00 „ 



55-00 „ 



140 



^th 



1 4-y, 



1 3 4 "" 



1 til 



2 9 5 0*'^^ 



,tll 



,t]l 



550000'" 



TTooooo 



I. fin 



All these 

 -eadilydis- 

 ^tinguish- 

 able. 



seen with a 

 little care. 



