[ 4t ]■ . 



The principal one, perhaps, is the quicknefs of the ftar's mo- 

 tion in the field of view. That this contributes to perception I 

 coUedl from hence ; that when a ftar, although within the field, 

 does not immediately prefent itfelf to the eye, as is fometimes 

 the cafe with Jupiter near the fun, if the telefcope be gently 

 moved, the apparent motion of the ftar immediately difcovers its 

 place. 



Another reafon may perhaps be the magnified image of the 

 ftar which thus becomes a more perceptible objeii, even though 

 its light is diminifhed ; this amplification, as far as I can perceive, 

 is nearly in the inverfe proportion of the aperture ; in very confi- 

 derable diminutions at leaft, it feems to follow this proportion : 

 thus with an aperture tV of an inch the polar ftar took 64." to 

 pafs the wire : with one of 7^ 32'\5 : but with -ps- 30", and 

 with 2,8 inches 19". 



I AT firft fufpedcd that this amplification arofe from fome 

 unavoidable error in the objed-glafs, which created a circle of 

 diffipation around the principal image, which circle became vifible 

 in proportion to the darknefs of the field ; but by the following 

 experiment it feems to arife from the inflexion of light. I 

 made a redangular aperture -^ of an inch broad, and A. long j 

 when this aperture was applied vertically, it gave me an elliptical 

 horizontal image ; and when applied horizontally, it gave me 

 an elliptical vertical image. Alfo when I applied a triangular 

 aperture, it gave an image nearly the fhape of a pear ; the broad 

 end being formed by the vertex, and the narrow end by the bafe : 

 the triangle was formed with a long bafe and fmall altitude, fo 



G - that 



