118 Messrs. Perry and Ayrton on 



great difficulties in the way of exact determinations of the 

 powers of lights. Our experience in experimenting with elec- 

 tric lights leads us to believe that, but for this difficulty, we 

 should have a vast body of information on the subject of elec- 

 tric lighting, instead of the vague and conflicting statements 

 which fill the scientific journals. Perhaps only those who 

 have made experiments will appreciate fully the great advan- 

 tages of our having some compact form of photometer. The 

 members of the Society will no doubt see many applications 

 of a compact photometer, such as measuring the light from 

 various parts of the sky for instance, in which ordinary pho- 

 tometric methods are unavailable. Now our instrument will 

 resemble a camera; and it may be turned in all directions. 



We were delighted to hear from Dr. Guthrie that his prin- 

 ciple of a "retention-image photometer," recently communi- 

 cated to the Society, has proved to be correct quantitatively. 

 The success of his test must be as interesting to physiologists 

 as to physicists. Besides testing his principle by using pow- 

 erful lights, he will have experiments to make concerning the 

 loss of light in reflection, as we shall have concerning loss in 

 refraction. Probably, however, he does not intend to apply 

 his instrument to powerful lights, on account of the very 

 great difficulty he would meet with in measuring the breadth 

 of the fine slot which would be needed. 



In ordinary photometric methods, the rays of light illumi- 

 nating unit area of screen, if coming from a powerful source, 

 are contained in a very small solid angle. We use the very 

 simple expedient of causing these rays to fill a much greater 

 solid angle by passing them through a thin concave lens, and 

 in this way obtain the same amount of illumination as before, 

 but on a screen placed at a short distance behind the lens. 

 Thus we not only save space, but prevent a great deal of the 

 absorption which occurs when light passes through air. This 

 absorption is sometimes very great in London. We have, 

 however, absorption in the lens as a disadvantage; but with a 

 very thin lens this may probably be reduced to an almost in- 

 appreciable amount. 



A is the light to be measured ; D is a paper screen illumi- 

 nated by light passing through the concave lens E. The 

 frame F, which carries the lens, slides on a stand Gr, which 

 has marked divisions, so that a pointer tells the distance from 

 the focus of the lens to the screen D. The sides of the space 

 F D are of black cloth, like the sides of a folding camera; and 

 the inside is all blackened except the screen D. A circular 

 plate with three round holes of different sizes is in front of 

 the lens. A diaphragm of this kind is not necessary, if we can 



