to the Undulatoty Theory of Light. 441 



My first object was the comparison of the illumination in 

 the dark and bright parts of Newton's rings as in Mr. Potter's 

 experiments ; but here there are several difficulties of no small 

 magnitude to be encountered : first, we must suppose equal 

 portions of each dark and bright space, isolated so as to be- 

 come the fair objects of comparison for the intensity of light 

 they respectively give. In the next place, there is the radical 

 difficulty in using the apparatus which Mr. Potter so candidly 

 allows, which arises from the necessity for transferring the 

 eye from the inclined glasses to the rings alternately. For all 

 these difficulties, indeed, as Mr. Potter has justly observed, 

 there is no remedy but steady and repeated practice. By this 

 means, but certainly by no other, can the eye become suffi- 

 ciently accustomed to the work to be able to form an un- 

 hesitating decision, and to give confident approximations, at 

 least, to correct comparisons. After a tolerably long experi- 

 ence in this sort of experiment, I must candidly own that I have 

 never been able to feel that entire and perfect confidence in 

 such determinations which would suffice for resting upon them 

 any very nice and delicate conclusions. Yet I agree with 

 Mr. Potter, that long practice will enable us to attain a de- 

 gree of precision which at first sight would have appeared 

 hopeless. 



I will not, however, dwell upon these points, but proceed 

 at once to detail my experiments, in which, with every degree 

 of care and caution, I have, owing to some cause at present 

 quite inexplicable to me, arrived at results very different from 

 those of Mr. Potter. 



It will not be necessary for me to detail the construction 

 of my apparatus, because it was in every point precisely the 

 same as Mr. Potter's, for which I would refer the reader to 

 his paper in this Journal, September 1832, [Lond. and Edinb. 

 Phil. Mag. vol. i.] p. 174. I will simply proceed to give the 

 results of my experiments. 



In this case the absolute areas of the two portions of in- 

 clined glasses are the same, or (using the accented letters to 

 apply to the second glass) a = a', and consequently by the 

 foregoing principles, we shall have (since d = d!) 



I i cos $ 



V " i f cos 4/ ' 



In order to find / and i' I have adopted Mr. Potter's valu- 

 able and curious formula for glass, which is, in fact, the basis 

 of all his calculations of these experiments; and I thus find 

 the following results, which will be best stated in a tabular 

 form. 



Third Series. Vol. 5. No. 30. Dec. 1834. 3 L 



