115' MEMOIR ON ACHROMATIC GLASSES. 



achromatic medium be brought nearer by two thirds of this 

 diftance, the double refraction will occafion only a feparation 

 Modeofufing of feven minutes between the images. Thus if we would 

 this micrometer. m eafure the diameter of an object, we mull move the achro- 

 matic medium toward the focus, till the two images of the 

 object are feen precifely in contact ; when, having the angle 

 of double refraction given, by previous experiment, the focal 

 . diftance of the object glafs, and the diftance of the achromatic 

 medium from the focus, the diameter will be given by the rule 

 of proportion. It is obvious, that great care mult be taken in 

 determining the angle of double refraction at firft with preci- 

 fion, as on this will depend the accuracy of all fubfequent 

 meafures with the inftrument. 



Having thus improved the conduction of this micrometer, 

 I found that the glafs prifms, intended to correct the difper- 

 iion of the prifm of rock cryftal, left a refraction more and 

 more perceptible in proportion as the double refracting medium 

 The inftrument was carried farther from the eye glafs. I then availed myfelf 

 farther im- f Beccaria's dilbovery, and cut my two prifms fo that the firft 

 two prifms of was m tne direction of the double refraction, and the fecond 

 jock cryfta) cut m that in which the double refraction is not perceptible : by 

 reak>ns! te theie means I na d a double refracting medium abfolutely ex- 

 empt from colours and refractions. I did not flop here, for I 

 The effect of was deiirous of extending the effect of the double refractions 

 the double re- f a8 to meafure the diameter of the fun ; and I accomplished 

 b l cd ' this, which appeared to exceed the known power of the dou- 



ble refraction of rock cryfial, which does not go beyond 

 twenty minutes, when it is cut in the moft advantageous prif- 

 hy means of matic fhape. . For this I employed two equal prifms, cut in 

 two equal the direction mod fuitable to my purpofe ; and on placing 



them in oppofite directions I found, that the double refraction 

 was not perceptible ; but on reverting their directions the 

 double refraction of each prifm was nearly doubled, fo that I 

 .obtained two images feparated by an interval of forty minutes. 

 This inftrument I ought not to omit, that in this new conftruction there are 

 dvfficult to exe- difficulties of execution not eafy to furmount, which may have 

 been one reafon why thefeinltrurnents, fo ufeful to navigators 

 and in certain very nice aftronomical obfervations, have not 

 Hence the au- been adopted. This induced me at length to adopt Euler's 



thor^asledto met h d. In the conftruaion of achromatic objeft glatTes I 



avail himfe'f of J 6 



Euler's method, found I could increa(e or diminifh the alfolute effect of the 



Effcdofthe 3 N double 



