40 



Microscopical Essays. 



The picture, formed by a convex lens, is either larger or lefs 

 than the object, in proportion as it's diftance from the lens is 

 greater or lefs than it's diftance from the objecl;. 



As an object becomes perceptible to us, by means of the image 

 thereof which is formed on the retina, it will, therefore, be feen 

 in that direction, in which the rays enter the eye to form the 

 image, and will always be found in the line, in which the axis of 

 a pencil of rays flowing from it enter the eye. We from hence 

 acquire a habit of judging the object to be fituated in that line. 

 Note ; as the mind is unacquainted with the refraction the rays 

 fuffer before they enter the eye, it judges them to be in the line 

 produced back, in which the axis of a pencil of rays flowing from 

 it is fituated, and not in that in which it was before the refraction. 



If the rays, therefore, that proceed from an object, are re- 

 fraaed and refleded feveral times before they enter the eye, 

 and thefe refraBions, or refkaions, change confiderably the' 

 original direaion of the rays which proceed from the object ; it 

 is clear, that it will not be feen in that line, which would come 

 {trait from it to the eye ; but it will be feen in the direction of 

 thofe rays which enter the eye, and form the image thereof on it. 



We perceive the prefence and figure of objeBs, by the im- 

 preffion each refpeaive image makes on the retina ; the mind, in 

 confequence of thefe impreffions, forms conclufions concerning 

 the fize, pofition, and motion of the objea. It mull however be 

 obferved, that thefe conclufions are often reified or changed 

 by the mind, in confequence of the effects of more habitual im- 

 Dreflions. For example ; there is a certain diftance, at which, 



