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LXXIX. Description of the Gyreidometer , an Instrument suited to 

 the exact Measurement q/" Ne\\i;on's Rings. By E. Wilde*. 



[With a Plate.] 



A FEW years ago an instrmnent was constructed by Jericau, 

 which, by giving a certain position to the glasses used in 

 the experiment, was peculiarly suited to the exhibition of the 

 coloured rings of Newton. This end was accomplished by giving 

 a plate of glass Avith parallel surfaces a small inclination towards 

 a moveable plane which contained the convex lens. The arrange- 

 ment permitted of the slow approach of the glasses, and the 

 alterations of the rings due to this approach were easily observed. 

 The instrument was named a ffyreidoscope. 



In the gyreidometer the above convenient arrangement of the 

 glasses has been substantially retained. To render the instru- 

 ment suited, however, not only to the exhibition of the rings, 

 but also to their measurement, a different arrangement in other 

 respects had to be resorted to. 



The following are the parts which go to the constraction of 

 the gyi'eidometer : a (Plate III. fig. 1) is a brass plate, in which 

 is placed a slide b, which can be moved by the micrometer- 

 screw c ; d is a plate of glass with parallel surfaces, under which 

 is placed the convex lens ; the plate is inclined at a small angle 

 to the horizontal path of the lens, and is held in this position by 

 the rest e and the screw/. To save the plate of glass from frac- 

 ture, the screw is caused to pass through a spring, which, when 

 the glasses are closely pressed, yields a little ; g and g' are two 

 arms capable of rotating in a vertical plane ; they carry the brass 

 plate //, which holds a second slide ; the latter can be moved by 

 the micrometer-screw k ; m is a mici'oscope fastened to this slide, 

 around the axis of which the entire plate h permits of being 

 turned ; n is an arch graduated to quarter degrees ; over it moves 

 a vernier, which, by means of the screw p, can be fastened to 

 the arm y. In order to be able to measure the angles of inci- 

 dence and reflexion to a minute, a space equal to 14 divisions of 

 the arch is divided into 15 parts on the vernier. The entii-e 

 apparatus, including the microscope, ought to be only about 7 

 inches long, the same in height, and 2| inches wide. 



The point O of the arch n, and the inclination of the plate of 

 glass to the path of the slide underneath, were determined in 

 my instrument by reflexion. By means of the screw e and of 

 three leveling-screws, the plate of glass and the lens were 

 brought into such a position, that when the slide containing the 

 latter was moved, no altei'ation in the colour of the rings followed, 

 thus pronng that the plate of glass was parallel to the path of 



* Translated from Poggendorff's Annalen, vol. Ixxxi. p. 264. 



