THE MIOKOSCOPE. 9 



tity of light by which every point of the object is rendered 

 visible. The naked eye, as shown in Fig. 2, admits from each 

 point of every visible object a cone of light having the di- 

 ameter of the pupil for its base, and most persons are familiar 

 with that beautiful provision by which in cases of excessive 

 brilliancy the pupil spontaneously contracts to reduce the cone 

 of admitted light within bearable limits. This effect is still 

 further produced in the experiment already described, of look- 

 ing at an object through a needle-hole in a card, which is 

 equivalent to reducing the pupil to the size of a needle-hole. 

 Seen in this way the object becomes comparatively dark or ob- 

 scure; because each point is seen by means of a very small 

 cone of light, and a little consideration will suffice to explain 

 the different effects produced by the needle-hole and the lens. 

 Both change the angular value of the cone of light presented 

 to the eye, but the lens changes the angle by bending the ex- 

 treme rays within the limits suited to distinct vision, while the 

 needle-hole effects the same purpose by cutting off the rays 

 which exceed those limits. 



It has been shown that removing a brilliant object to a 

 greater distance will reduce the quantity of light which each 

 point sends into the eye, as effectually as viewing it through a 

 needle-hole; and magnifying an object by a lens has been 

 shown to be the same thing in some respects as removing it to a 

 greater distance. We have to see the magnified picture by the 

 light emanating from the small object, and it becomes a matter 

 of difficulty to obtain from each point a sufficient quantity of 

 light to bear the diffusion of a great magnifying power. We 

 want to perform an operation just the reverse of applying the 

 card with the needle-hole to the eye we want in some cases to 

 bring into the eye the largest possible pencil of light from each 

 point of the object. 



Eeferring to Fig. 3, it will be observed that if the eye could 

 see the small arrow at the distance there shown without the 

 intervention of the lens, only a very small portion of the cones 

 of light drawn from its extremities would enter the pupil; 

 whereas we have supposed that after being bent by the lens the 

 whole of this light enters the eye as part of the cones of smaller 

 angle whose summits are at C and D. These cones will further 



