IN OPTICS. 85 



be seen to move the same way as the finger, 

 when this successively intercepts the rays pro- 

 ceeding from the image to the eye*. 



* Scheiner observed a fact of the like kind (Fundamentum 

 Opticum, p. 33) namely, that, if a small hole, made in any 

 substance, be held near to the eye, and an opaque body be 

 passed between them, from right to left, the left side of the 

 hole will first disappear. Mr. Grey afterward took notice 

 (Philosoph. Transact. Vol. xix. p. 286) that a needle he em- 

 ployed in this experiment was seen inverted ; from which he 

 supposed that the hole, or something in it, produced the 

 effect of a concave speculum. Mr. Harris, however, says 

 (Treatise of Optics, p. 141 ) that it is not the needle, but its 

 shadow on the other side, which is seen, and is the cause of 

 the inverted appearance. But the truth is, that the hole is 

 to be regarded as a luminous point, the rays of which fall 

 upon the retina before they are collected to a focus ; and 

 hence that the same appearances must be here observed as 

 in the experiment of Mr. Le Cat. In proof of this it may 

 be mentioned, that if the hole be placed at such a distance, 

 that the eye may refract its rays accurately to a point on the 

 retina, ; :no shadow or image of the needle will be seen j that 

 if the hole be still farther removed, and the eye be adapted 

 to a less distance, the shadow or image will again appear, 

 but its position will now be upright, and its motion the 

 same way as that of the needle itself $ and lastly, that, at 

 one given distance of the hole, either no shadow will appear, 

 or it will be seen upright, or it will be seen inverted, accord- 

 ing as the eye may be made to assume different states with 

 respect to its power of refraction, 



