Memoir on achromatic glasses. ]13 



By the Remitters of the Academy of Sciences, January the Grateloup ufed 

 • . ^ i j , j xi r maftic between 



6th, 1788, it appears, that Grateloup adopted the lame me- the g | iffejj 



thod. He did not make ufe of a fluid interpofed between the 



glafles, however, but of maftic. This fubftance had long been 



in ufe among jewellers, for cementing together ftones, which 



it did in fuch a way, that it was fcarcely poflible to diftinguilh 



the two (o united from a Tingle ftone ; and hence Grateloup 



conceived, it would render his object glafles as it were one r 



folid piece, without being liable to evaporation like a fluid, 



while it corrected the errors of their interior furfaces. Glafles But change of 



cemented together with maftic, however, do not anfwer, at 2d tSiMfet^i- 



Ieaft for fea voyages, as change of temperature, and the fea injure maftic. 



air, affect the maftic very much. On this account I have pre- Refin therefora 



ferred refin, and even turpentine the moft fluid and tranfparent P referab J e > or 



. turpentine. 



I could get. Chemifts perhaps may find fubftances ftill pre- 

 ferable to thefe, and it is an object certainly worthy their at- 

 tention. Dr. Blair tried a great number of folutions of metals Dr. Blair tried 

 and femimetals, and he fays, that certain falts, particularly J^nces* f" 

 fal ammoniac diflblved in water, give it a confiderable power falts, and oxi- 



of difperfion. The oxigenated muriatic acid, too, poflefles &£** muriatic 

 r .. ? , r . , acid, had a con- 



this quality in a great degree ; but butter ot antimony has a fiderable power 



ftill greater effect, for one prifm of this is equal to three of of difperfion; 



crown glafs of fimilar dimenfions. Dr. Blair conftructed an antimony had 



object glafs with crown glafs and butter of antimony, but he ft iN more; but 



obferved, that it occafioned irradiations, which led him to irradiations 



prefer oil of turpentine or other eflential oils *. whence he pre- 



The ufe of achromatic glafles applied to graduate circles or "" e cn 

 fegments of circles for meafuring angles, and the defects to 

 which inftruments of this kind are liable, may be pafled over 

 as not to the prefent purpofe, which is the beft mode of mea* 

 Airing very fmall angles. 



Buffon, who had paid fome attention to the formation of Bufflm Uft 



rock cryftals, finding no indication of a double refraction in the P efted the 



...,,, r double refracliort 



experiments which I made on that of Madagafcar, in 1770, f the rock cry f- 



thought this tranfparent quarter to be of a diffeient nature from tal of M ad agaf- 



the other cryftals ; but before he made up his mind on the fub- been overlooked. 



ject, he defired me to examine it afrefh with a view to this 



property. 



* An ample abridgement of Dr. Blair's paper is given, Phil, 

 journal, 4to, I. 1. 



Vol. IV. — February. I It 



