VOL. LXVII.] PHILOSOPHICAL TRANSACTIONS. 249 



without any knowledge whatever of my ideas on the subject ; he has been the 

 first who announced it to the world, who had it executed, and who made use of 

 it : I have therefore no pretensions whatever on that head ; he has the merit of 

 a great discovery, and astronomy has the sole obligation of it to him. But the 

 Abbe Rochon has only made use of the double refraction of the rock crystal for 

 his micrometer, and I am assured he has said, that his prism could give him no 

 more than 6 degrees. Now it is well known, that pieces of rock crystal, large 

 enough and pure enough for these purposes, are extremely rare ; besides, the 

 difficulty of working them is great, that substance being harder than glass, and 

 requiring the utmost attention in cutting, in order to obtain the difference 

 desired between the 2 refractions. I think therefore, that it will be doing an 

 essential service, to propose another micrometer of common glass, to explain the 

 theory of it, and to extend it to much larger angles, which may render it appli- 

 cable to the optical instruments made use of in the navy, in taking geographical 

 latitudes and longitudes. 



I had already made a prism of this sort, and showed the Abbe Fontana its 

 effect for the double image of the sun on his excellent little achromatic glass : 

 the 1 images were procured by applying this prism to the object-glass with the 

 hand, in such a manner that it covered only 4- of it : pushing it more or less 

 forward, occasioned a change in the brightness of the light of the 2 images, 

 and showed that they might be made equally clear. By changing the inclination 

 of this piece, the distance between the 1 images was varied, which did not alter 

 when its distance from the object-glass was varied without the glass. This piece 

 was a common prism, which gave a refraction a little greater than the apparent 

 diameter of the sun : I added another to it afterwards, of the same kind and 

 equal, both of them having circular bases. Turning one of the 2 parts on its 

 axis, will vary the angle from O, to double each in particular, which occasions 

 the 2 images to approach to, or recede from, each other. A much slower varia- 

 tion is obtained by the greater or less distance of the prism from the object-glass; 

 but there is a particular reason for which one cannot give it too large a one, the 

 contraction of the pencil of rays belonging to each point of the object, not al- 

 lowing that distance to be very great, for fear of weakening too much the direct 

 image towards the middle of the field, by the interception of too great a part of 

 the same pencil, which in the end would occasion its being altogether lost. 



There is now making for me a rude machine, in which one of the pieces may 

 be turned by the hand on its axis, to make the distance between the 2 images 

 somewhat larger than that which is intended to be measured, as for instance the 

 diameter of the sun ; and by the help of a moveable screw, one may carry the 

 prism, thus composed, to a distance from the object-glass, by a motion similar' 

 to that of the small mirror of the telescope. I have had it adapted to an ordi-- 



VOL. XIV. K K 



