436 Prof. Powell's Ftn-ther Remarks on ExjJt'iimcnts 



easily compared with the stripes torined in air by ai'ranging 

 the lens so that one half of it is exposed to the direct rays, 

 the other to those passing through the medium. The two 

 portions of the stripes thus seen are, in fact, continued exactly 

 in the same straight lines ; and the eve would be well able to 

 judge of the smallest deviation in such extremely delicate and 

 well-defined lines, did any exist. The stripes, however, are 

 precisely of the same breadth, or the values of c the same, 

 whether with or without the interposition of the medium : 

 hence, as before, A must be diminished exactly in proportion as 

 m is increased. But these experiments refer only to the fact, 

 that along the length of what we term a ray of light, there 



occur at intervals equal to the value of-—, points at which 



the ray is in some way affected with a different character, such 

 that the concurrence of two rays at a point of like affection 

 shall produce light, but atone of opposite affection, darkness ; 

 this character evidently changing gradually from one condition 

 to the other. All this, however, may consist with the notion 

 of these different affections occurring simply at successive 

 fixed points equidistant along the ray, which are merely more 

 crowded together in a denser medium. It is then a separate 

 conclusion, established by distinct evidence, that these intervals 

 are propagated along the line which represents the ray with 

 a definite velocity, diminished in proportion to the refractive 

 power of the medium. The simple fact of the diminution of 

 the intervals is, however, usually presented mixed up with the 

 consideration of this progressive propagation, or rather of its 

 retardation in denser media, in explaining the shifting of the 

 stripes, as in the experiments referred to in my former paper, 

 and to which the method I have suggested by means of a 

 thin prism, offers a convenient auxiliary process ; showing at 

 once the existence of the effect and its amount. Connected 

 with this subject, some curious researches were given by Mr. 

 Potter, in a paper read before the British Association for the 

 Advancement of Science, at the Oxford meeting in June last, 

 to the publication of which I look with great interest, espe- 

 cially as they were founded on my experiment just alluded 

 to, and as their tendency appeared to be somewhat at vari- 

 ance with the received views on this point*. 



There are many students to whom it is an important object 

 to know what is the least possible amount of apparatus in- 

 dispensable for carrying on their experiments. It may be 

 desirable to mention, therefoi'e, that in order to perform the 



* Mr. Potter';; paper here alluded to will appear in our next Number.— Ed. 



