248 PHILOSOPHICAL TRANSACTIONS. [aNNO 177/. 



particular zero, but also its particular curve, whose ordinates will always measure 

 the equations applicable iii the respective situations. The equatorial curve will 

 probably change the fastest, and the others become gradually flatter, as they 

 approach towards the poles, where the greater, but more uniform density of the 

 atmosphere may occasion it to differ but little from a straight line. I apprehend 

 however that even at the pole some small diminution might be found to take place 

 in the equation, were it possible, in that region, to prove it by experiments at a 

 sufficient height above the level of the sea. 



XXXV. Of a Nt7v Micrometer and Megameter. By the Abbe Boscovich, &c. 



p. 789. 



Having heard that the Abbe Rochon had exhibited a kind of micrometer, 

 which, by means of a prism of rock crystal whose angles could be varied, gave 

 2 images of the same object, and changed their distances by the circular motion 

 of one of the 2 parts which composed it ; I told some of my friends, and 

 among others the Abbe Fontana, that 1 saw how the thing was done ; but that 

 an improvement would be made in it if the distance from the prism to the focus 

 of the eye-glass were made variable I added that the same effect might be pro- 

 duced without the double refraction of the rock crystal, with a prism made of 

 common glass, only smaller than the aperture of the object-glass. The rays 

 which pass through the prism would then form an image which would be seen 

 out of its natural place ; and those which pass without would give ancither image, 

 in the same place it would have appeared in, if the prism had not been there. 



A few days after, the Abbe Fontana was told, that the Abbe Rochon had 

 thought of another micrometer with a prism of rock crystal, which, approaching 

 more or less to the focus of the object-glass, had the advantage of producing a 

 very great effect, and might be acromatic ; that having accordingly had his in- 

 strument executed, and made several observations with it, he had prepared a 

 paper on the subject, to be read at the next meeting of the academy. This the 

 Abbe Fontana immediately told me, and I repeated what I had said to him the 

 first time on the effect of this micrometer ; adding at the same time the precise 

 measure of the scale for the measure of the same effect, and the facility of ob- 

 taining the same thing without the rock crystal ; mentioning likewise other ad- 

 vantages which might be derived from the common glass prism not covering the 

 whole aperture of the object-glass ; and, among others, that of being able to 

 measure much greater angles by this means than by the double refraction of the 

 rock crystal. 



The Abbe Rochon accordingly read his paper to the Academy, and mention 

 has been made of it in the public prints : he has therefore the merit of having 

 thought of the same thing, at the same time with, or perhaps before me. 



