4 Trait suctions of the 



fectly bright linear images without the errors of refraction from four 

 surfaces and the changing film of Canada-balsam cement. We got 

 a nearly perfect surface to engrave upon, the plano-convex lens 

 being far easier of true workmanship than the formation of exactly 

 parallel surfaces. The consequence was that he got a brilliant field, 

 brilliantly marked, with an unchangeable micrometer. That was the 

 first point. 



The writer had just tried rather a curious experiment. If you 

 place the micrometer in your different eye-pieces, say four ; then view 

 the stage micrometer, the Fellows would be surprised to hear that the 

 reading of the micrometer as to the size of a 100th of an inch on 

 the stage, was the same for all the four eye-pieces, very nearly 

 indeed. It seemed very odd, for instance, that, under a J-inch 

 objective, he read 37 hundredths : placed another eye-piece B he 

 read 38 hundredths; in the D eye-piece he read 39 hundredths. 

 The conclusion was this, that if you put the same micrometer into 

 the four eye-pieces, ABCD, they all showed the same reading in 

 the size of a 100th of an inch upon the stage, provided the principle 

 of their construction and arrangement was precisely the same.* 



The new micrometer, engraved on a very long focussed plano- 

 convex instead of the old-fashioned cemented micrometer, contained 

 one hundred divisions to the inch, the same as the stage micrometer. 

 When the eye-lens of the eye-piece is exactly 10-llths of an inch, 

 then, and then only, did the reading of the new micrometer, as 

 compared with a 100th on the stage, give the magnifying power of 

 the whole instrument in use.t If anyone looked through the micro- 

 scope by the use of this micrometer, he could read instanter the 

 magnifying power, no matter what objective was employed. It 

 seemed very strange at first sight that all the eye-pieces should 

 read nearly the same. But only one of them precisely gave the 

 correct magnifying power. If the divisions of the new lens en- 

 graved micrometer now exhibited were 200 to the inch, then only 

 an eye-piece having a ^-inch focal eye-lens would tell the power at 

 sight, and so on.t He must compliment Mr. Acland upon the very 

 beautiful manner in which the "lens-micrometer" now used in 

 the eye-pieces had been engraved. He had often tried some experi- 

 ments in reference to cutting fine lines upon glass, and had found 

 that, by turning round the writing diamond upon its axis, there 

 was generally some particular angle of rotation at which the diamond 

 ploughed most beautiful grooves, scattering minute little glass curls 

 or shavings, and then the grooves were as clear as if they had been 

 planed a thousand times larger. 



* This principle is in general that the field-lens shall have three times the 

 focal length of the eye-glass, and that the interval between the glasses shall, to 

 preserve the achromatism, be exactly one-half the sum of their focal lengths. 



t Provided the distance of distinct vision is 10 inches. 



X Accurately 7 s r for 10 inches vision. 



