794 -Abbe boscowich’s Account of 
when its diftance from the objeCt-glafs was varied with- 
out the glafs. This piece was a common prifm, which 
gave a refraction a little greater than the apparent dia- 
meter of the Sun : I added another to it afterwards, of 
the fame kind and equal, both of them having circular 
bafes. Turning one of the two parts upon its axis, will 
vary the angle from o, to double each in particular, which 
occa lions the two images to approach to, or recede from, 
each other. A much flower variation is obtained by the 
greater or lefs diftance of the prifm from the objeCt-glafs ; 
but there is a particular reafon 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 allowing that 
diftance to be very great, for fear of weakening too much 
the direCt image towards the middle of the field, by the 
inter- 
tion en changeant, hors de la lunette, fa diftance a l’obje&if. Cette piece etoit 
un feul pFiftne limple, qui donnoit une refra&ion un peu plus grande que le 
diametre apparent du Soleil : j’y ai fait ajouter apres un autre femblable et egal ; 
l’un et l’autre ayant les bafes circulates: en tournant fur fon axe Tune des 
deux parties, on variera l’angle depuis zero, jufqu’au double de chacun en parti- 
culier, ce qui fait approcher et eloigner les deux images entre elles: on obtient 
une variation beaucoup plus lente par l'eloignement plus grand ou plus petit 
du prifme a Ifobjedtif; mais il y’a une raifon particuliere pour laquelle on ne 
peut pas lui en donner un trop grand; car le retreciffement du pinceau de rayons 
appartenans a chaque point de l’objet, ne permet pas de l’en eloigner trop, ce qui,. 
versle milieu du champ, aftbibliroit trop Fimage dire&e, eninterceptantune trop 
ffrandc. 
o 
