48 



Professors Ayrton and Perry on a 



mirror about its horizontal axis necessary to enable measure- 

 ments to be made of rays coming at any angle, after measure- 

 ments have been made of the horizontal beam, is exactly equal 

 to the inclination of the beam in question. 



Using Bumford's method in this latest, form of our photo- 

 meter, Ave are to a great extent independent of the presence of 

 other sources of illumination of the screen, so that the appa- 

 ratus need not be enclosed in a box. At the same time, how- 

 ever, the sensibility of the test is much increased by placing 

 a shade to prevent* the electric light shining directly* onto the 

 screen. On this screen of blotting-paper, B, is thrown the 

 shadow of a black rod, A, placed in front of it, by a candle in 

 the candle-holder, D. 



Now it is well known that if an electric light is also allowed 

 to illuminate this screen, and to throw a second shadow of the 

 rod A on the paper, and if the candle is adjusted at such a 

 distance that the two shadows are of equal intensity, the 

 strength of the light is to that of the candle in the ratio of the 

 squares of their distances from their respective shadows. But 

 instead of allowing the strong light to pass directly to the 

 screen, we cause it to pass through the concave lens in the 

 sliding wooden frame C. A pointer on this slide tells the 

 distance of the lens from the screen. As you are all aware, 

 the weakening of light-intensity produced by the lens enables 

 us to leave our electric lamp within a few feet of the instru- 

 ment. We have experimentally found that there is no appre- 

 ciable loss of light in passing through the lens. The candle 

 slides on the bar J; and its distance from its shadow is shown 

 by a pointer on a scale. If/ is the focal length of the lens, 

 D the distance of the electric light from the paper-screen, d 



