MEMOIR ON ACHROMATIC GLASSES. H5 



it. Hence it follows, that in cutting a lens of rock cryftal for The refraction 

 • i r • 11 i i l r r of the cryftal is 



optical purpoles we mult take pieces parallel to the bale of f lng i e j n t h c ^ 



the cryftal, fo that the axis of the lens coincides with that of rettionof its 



the cryftal, or is at Ieaft parallel to it. Thus far Beccaria. 



When we would apply to practice this properly in rock Aprifm of rock 



cryftal of doubling the images of obje&s, the colours thatarife cr y fta ' m " ft be 



J b b J , corrected byone 



from the prifmatic figure neceflary to be given to it rnuft be of glafs : 



corrected by a glafs prifm, or the images will not appear well 



defined. Thefe two prifms united together may be called a the two forming 



double refraains achromatic medium, but in this ftale it can ? d * ub [ e re f™&- 

 r w g achromatic 



ferve only to meafure the fmall angle given by the double re- medium. 



fraction : yet it is eafy to conceive, that on applying two inftrument (or 



double refracting mediums one on the other, we may vary the meafu ring fmall 



effect of the double refraction at pleafure by a circular move- f two ofthefe 1 ' 



ment, as in the inftrument for meafuring colours which I laid mediums. 



before the Academy. Thus this double refraction, which has 



been deemed detrimental to the conftruction of optical inftru- 



ments, is in fact advantageous for the menfuration of fmall 



angles, as I convinced the Academy, February the 25th, 



1777, by an inftrument conftructed on this principle, which 



gave the meafure of fmall angles with a degree of precifion, 



for which we could fcarcely have hoped. This inftrument, its defect* 



however, had the inconvenience of giving four images of one 



object, which occafioned a confiderable lofs of light; and the 



lefs light we have, the lefs the accuracy with which the point 



of contact can be obferved. 



Endeavouring to remedy this imperfection, I foon difco- This remedied 



vered a more fimple conftruction for the inftrument, which I h ? ufingone 

 i-iir-iAi i r ti- i medium, made 



laid before the Academy the lame year. In this only one moveable along 



double refracting achromatic medium is required, which is tilc axls °* tlw 

 made to move along the infide of any telefcope in the axis of 

 the object glafs. The value of the double refraction is firft to 

 be determined by experiment, as in the former micrometer, 

 the achromatic medium being placed againft the object glafs. 

 It is then to be moved from the object glafs toward the eye 

 glafs. The angle of the double refraction will now be what 

 it was in the former fituation, but the feparation of the images 

 will be in the ratio that the diftance of the refractive medium 

 from the focus bears to the focal diftance. Suppofe for ex- 

 ample the double refraction to be twenty-one minutes, and the 

 focal diftance of the object glafs to be three metres: if the 



1 2 achromatic 



