420 



LECTURE XXXVI. 



ON OPTICAL INSTRUMENTS. 



Among the great variety of instruments depending on optical principles, 

 it is more consistent with our plan to attend first to those which may be 

 denominated optical measures, which are calculated either for the determina- 

 tion of the quantity or intensity of light itself, or for the examination of the 

 properties of various material substances with respect to light. Reflecting 

 quadrants and circles, which are often used in astronomical and nautical 

 observations, although they derive their utility in some measure from optical 

 laws, may most pr.operly be considered as belonging to the subject of practical 

 astronomy. 



It is a problem of frequent occurrence in economical investigations, to 

 compare the intensity of the light afforded by any two luminous objects. 

 For this purpose, it is necessary to assume as a principle, tliat the same 

 quantity of light, diverging in all directions from a luminous body, remains 

 undiminished at all distances from the centre of divergence. Thus, we must 

 suppose that the quantity of light falling on every body is the same as would 

 have fallen on the place occupied by its shadow: and if there were any 

 doubt of the truth of the supposition, it might be confirmed by some simple 

 experiments. It follows that since the shadow of a square inch of any sur- 

 face, occupies, at twice the distance of the surface from the luminous point, 

 the space of four square inches, the intensity of the light diminishes as the 

 square of the distance increases. We can judge with tolerable accuracy of 

 the equality of two lights by the estimation of the eye, but we cannot form 

 any idea of the proportions of lights of different intensities: if, however, 

 we remove two sources of light to such distances from an object, that they 

 may illuminate it in equal degrees, we may conclude that their original 



