M. E. Wilde on the Ch/reidomete?: 551 



the slide. The convex lens was then removed, and a coat of tin- 

 foil was laid upon the under surface of the plate of glass. The 

 lenses were now removed from the microscope, and the latter 

 was so placed that the reflected images of its top and bottom 

 openings appeared concentric ; the point of the arch was taken 

 for this vertical position of the microscope. The latter, by means 

 of the vernier, was then moved 34^ minutes beyond O, and the 

 plate of glass was so placed that the microscope stood perpen- 

 dicular to it as before. By this means an inclination of 34i 

 minutes was given to the glass ; and this angle being deter- 

 mined, the tinfoil was removed from the surface of the latter. 



A hundred revolutions of the under micrometer-screw causes 

 a horizontal motion of one inch ; the head is divided into 100 

 parts, every one of which therefore corresponds to a motion of 

 the slide equal to 0-0001 of an inch. This length of 0-0001 of 

 an inch is represented by ab (fig. 2) ; hence while the point a of 

 the slide moves onwards to b, the summit of the lens approaches 



the surface of the glass above it by the distance df= ^ „_„^ . 

 o J J 10000 



Tlie tangent of 34i' is =0'01 ; and hence for every division of 



the head of the screw c when tui'ned, an approximation of the 



glasses equal to 0000001 of an inch is effected. To obtain this 



proportion, we have chosen the particular inclination 34^' of the 



plate of glass to the slide underneath. 



In like manner 100 revolutions of the upper micrometer-screw 

 k go to an inch ; the head of this screw also being divided into 

 100 parts, by means of the fine hair which stretches across the 

 middle of the field of view of the microscope, and which, when 

 the screw is turned, moves along the diameters of the rings, the 

 length of these diameters can be measured to 0*0001 of an inch, 

 and can with tolei-able certainty be estimated to O'OOOOl of 

 an inch. 



The care with which Newton 200 years ago observed the rings 

 formed between two glass lenses is truly deserving of admiration, 

 the more so as his measurements were made with a pair of com- 

 passes and with unassisted eyes. On this account he found him- 

 self compelled to choose lenses with radii from 40 to 50 feet in 

 length ; for as the diameters of the rings stand in the ratio of 



~j , where (f) and <f>' denote the radii of curvature of the 





lenses, when the values of <}> and <f>' are so great as above given, 

 the diameters of the rings themselves must be considerable ; and 

 in this way an accurate measurement of them, even with imper- 

 fect apparatus, is rendered ])ossible. It requires, however, the 

 utmost care and trouble to prepare glasses of this description. 



