380 Intelligence and Miscellaneous Articles. 



African lakes, and to be due to climatic change. He brings forward 

 evidence in favour of the coincidence of change of level and climatic 

 change, but does not believe that his views are by any means con- 

 tradictory to those of Mr. Carson, for the phenomena may be 

 explained by a combination of the influences of climate with those 

 of mechanical agencies. 



9. "On Cheilostomatous Bryozoa from the Middle Lias." By 

 Edwin A. Walford, Esq., F.G.S. 



XLI. Intelligence and Miscellaneous Articles. 



ON A PHOTOMETRIC METHOD WHICH IS INDEPENDENT OF COLOUR. 

 BY OGDEN N. ROOD. 



rPHE principle underlying most of the photometric methods now 

 -*- in use depends on a comparison of the illumination of two 

 adjacent fields ; in some instances, as where a spot or a ring is 

 made to disappear, this idea is thinly disguised, but instantly 

 becomes apparent when the lights are differently coloured. 



The object of the present paper is to present a mode of pro- 

 cedure in which the coloration of the two surfaces, even though at 

 a maximum of vividness, is a matter of entire indifference, since 

 the process depends not on a comparison of these surfaces, but on 

 the shock which the retina experiences when one surface is quickly 

 withdrawn and replaced by another. If we take a uniform circular 

 disk of any colour, illuminate it evenly, and then set it in rotation 

 slowly or rapidly, the retina will receive no shock, since the parts 

 replacing each other in the retinal image are in all respects identical ; 

 in other words, no flickering will be perceived. If, however, one 

 half of the disk reflects less light than the other by -^ of the total 

 amount, with appropriate rates of rotation a faint flickering will 

 be noticed, which will increase in intensity as the difference of 

 the luminosities of the two halves of the disk is made greater. 



Accordingly I prepared a set of about a hundred disks of drawing- 

 paper, their depth of tint ranging from the whitest paper down 

 to the blackest, the gradation being as even as practicable. For 

 the determination, then, of the reflecting-power, for example of a 

 vermilion disk, it was only necessary to select from the series a 

 grey disk which, when combined with it in equal parts, gave no" 

 perceptible flicker, and afterwards to determine the reflecting- 

 power of this grey disk in terms of the standard white cardboard. 

 Results were thus obtained for the principal colours — red and 

 blue-green, yellow and blue, green and purple. The difficulty in 

 measuring the reflecting-power of these coloured disks was in all 

 cases the same, and was due to the fact that it is hard to obtain 

 grey disks with absolutely uniform surfaces. This trouble, or a 

 lack of uniformity in the coloured disks themselves, was to a con- 

 siderable extent obviated by reversing the halves of the composite 



