196 Scientific Proceedings^ Roi/al Dublin Society. 



tliank my assistant, Mr. W. "Warwick, b.a., for repeating tliese observations, 

 and also for other assistance in tliis paper : — 



Silvered mirror, . . . . . . 100 



Plane glass surface, . . . . . . 65 



Ground glass, . . . . . . 45 



White card, . . . . . . . . 45 



Grey card, . . . . . . . . 35 



Dark grey card, . . , . . . 21 



Smooth black paper, . . . . . . 20 



Black cotton cloth, . . . . . . 16 



Dull black woollen cloth, . . . . 5 



As light is scattered at all angles, tlie reflecting surface can be more 

 or less inclined, and the relative reflecting powers at different incidences 

 thus determined. When the scattered light is coloured, the exact amount 

 scattered can be determined by interposing a suitably coloured glass screen 

 when the silvered mirror is used, and comparing the depth of liquid then 

 required for extinction with tliat required when the light is scattered from 

 the coloured surface. 



If, instead of finding tlie depth of the neutral tinted liquid required to 

 obscure the reading of the test type, we measure the depth required to 

 extinguish the light scattered from various surfaces, the remarkable fact is 

 revealed by these preliminary experiments, that even with dark surfaces, 

 such as black paper or cloth, the intrinsic brightness is not reduced much 

 more than 40 per cent., though tlie illuminating power is reduced 80 to 90 

 per cent. This subject requires, and I hope shortly to give it, further 

 investigation. The apparatus also affords a means of measuring tlie relative 

 sensibility of the fovea and of the retina immediately surrounding it to 

 (i) the light sense, (ii) the colour sense, and (iii) the form sense or visual 

 acuity. 



Furthermore, this method of absorption enables one to determine with 

 accuracy the somewhat difficult problem of the relative value of different 

 systems of lighthouse illumination ; for, as already shown, it measures the 

 perceptive power of the eye for any given light, and this depends on the 

 brightness, as distinguished from the illuminating power, of different sources 

 of light. 



The theoretical law which expresses the absorption of liglit by a homo- 

 geneous medium of varying thickness is well known. In the particular case 

 of the foregoing instrument, as every successive increment in the depth of the 



