174? Mr. Potter oyi a neso Photometer by Comparison, and 



noniical purposes we merely change its direction, taking it 

 awav from one place and throwing it upon another ; and in 

 all such operations light is invariably lost : but if we can sti- 

 mulate the retina and render it more sensible to a weak light 

 by the mode of its application, we obtain the very same effect 

 as if we had used a more powerful beam. The experiments 

 which I have made ou this subject have been more successful 

 than I could have expected ; and I hope to be able on some 

 early occasion to submit them to the Association. 



XXXVIII. On an Instrumeiit for Photometry hy Comparison, 

 and on some Applications of it to important Optical Phe- 

 nomena. By R. Potter, Jun. Esq.* 



WHEN engaged in examining the phaenomena of the co- 

 lours of thin plates in the form of what are generally de- 

 nominated Newton's rings, I was surprised to find that the rings 

 were so distinct in the transmitted light, and particularly when 

 homogeneous light was used. These rings are now generally 

 allowed to be produced, by the agency of the light which has 

 been twice reflected at the surface of glass, under an incidence 

 very nearly perpendicular. 



Photometry had taught me that most of the common sorts 

 of glass reflect about J^yth of the light incident upon them in 

 this case ; and we should expect two reflections to give an in- 

 tensity of Jyth of ^'^yth, or y^o^'^ of the first intensity. Now it 

 requires very little consideration to see, that the presence or 

 absence of so small a quantity of light is quite beyond detec- 

 tion by the eye, and experiments ai-e easily executed by which 

 it may be proved. 



The difficulty of accounting for so great an effect being 

 produced in a pencil of light, by so small a proportion of it, 

 renders almost equally inadmissible every hypothesis which 

 has yet been proposed to account for the whole pha;nomena 

 of thin plates. On the doctrine of fits of easy reflection and 

 transmission, which many experiments, otherwise, show must 

 be dispensed with, even in the theory that light is caused by an 

 emitted matter, this difference of the intensity of the dark and 

 bright rings in the transmitted light is not to be satisfactorily 

 accounted for. The theory that light consists of undulations in 

 a subtile aether, gives scarcely any more admissible reason for 

 the lahole appearances, than the other. For in this theory the 

 intensity of the light being taken as the amount of t'/s viva in the 

 vibrating molecules, the modification which could be intro- 



♦ Communicated by the Author. 



