THE MICROSCOPE. 



Again, with an uncovered Rogers' plate (and probably with the 

 draw tube at a different point from that previously taken): 



jyL-y in. equals 1.58 turns, collar at 0° 



1.575 " " o" 



1.62 " " 8°.2 



1.59 " 



(As the lines were protected by a ring of varnish the covers did 

 not lie quite flat.) It is evident, then, that the magnifying power is 

 changed quite sensibly by the use of the cover-correction. Hence 

 in accurate work the cover-glass over the micrometer and over 

 the specimen should have the same thickness. 



3. It is sometimes convenient to know the thickness of a 

 cover-glass; this may be found quite closely by means of an objec- 

 tive with correction. Taking the covers used above, and having 

 focussed on dust or finger marks on the under side turn the collar 

 till dust on the upper side is in focus; with the thinner glass several 

 trials gave as the reading of the collar t,''.6, 3". 6, 3°. 75, etc., working 

 backwards focussing on the top with the collar at 9°. 6 and 

 then on the lower side by the collar the reading was 6°.i 

 twice, a change of 3.5 : mean of 7 trials gave 3".56: similarly 

 with the thicker cover, mean of 5 trials gave 7°. 58. If we as- 

 sume the change of the collar to be just proportional to the 

 thickness of the glass, since the thin glass is .0058 inch we 

 should have 3.56: 7.58:: .0058: thickness of thick cover: solving we 

 find it to be .01235 inch — a difference of less than yoVo "^^^ from 

 that found by a Brown and Sharpe's gauge — a quantity scarcely 

 measurable with this gauge. If one has, then, a single cover-glass 

 whose thickness is known, by a simple proportion the thickness of 

 any other one can be found in a moment. For this particular lens 

 the reading of the collar multiplied by 1.6 will give very closely the 

 thickness in thousandths of an inch. Makers might easily furnish 

 for their lenses the constant multiplier to be used as this 1.6 is; or 

 divide the scale so as to indicate directly the thickness in thousandths 

 of an inch. 



It may be added that to get the micrometer readings given, the 

 utmost care was needed, for I could find no stand firm enough to 

 resist the unavoidable slight pressure of the finger on the head of 

 the micrometer screw; further, in one well known stand the tube was 



";< 



