78H 
DR. OLIVER LODGE OX ABERRATION PROBLEMS. 
Now, as there is no hypothesis or difficulty whatever about the aberrational effect 
of a moving telescope, all that has been said of a grating applies, at least broadly, to a 
prism. 
Effect of Motion on the Disjoei'sion of a Prism. 
57. The deviating })Ower of a prism depends on its relative refractive index with 
resjiect to the surrounding medium; hence, in this sense, its deviation is certainly 
affected by the length of the waves with which it is supplied. 
Its dispersive power, however, is not a superficial, but a deep-seated, phenomenon, 
depending on its internal structure ; and, since no variation of outside medium can 
affect internal wave-length, the dispersive power of a jndsm may be assumed constant 
for given waves. It follows that the dispersion caused by a given prism, immersed 
in different media, is simply proportional to the mean deviation in each case for given 
kind of light. 
But what about the effect of motion t 
If only we can assume that the prism interferes with the ether as little as the 
grating has been supposed to do, then all that has been said of the grating remains 
true of the prism. If we supposed the prism to modify the free ether inside it, 
we should have to modify this statement. On the hypothesis of Fresnel, however, 
the free ether is not supposed to be affected; and experiments directed to test 
the matter, by ascertaining the effect of prism chasing a source at the same speed, 
have resulted in finding this effect zei^o, in accordance with the above statement. 
Hence it must be allowed that a Doppler effect observed by a prism depends really 
on wave-length, but apparently on frequency, just as is the case with a grating. 
It must be noticed that the observation of a Doppler effect by a prism depends 
entirely on dispersion ; i.e., on waves of different length being affected differently. 
But prisms can be constructed whose dispersion is corrected and neutralized. Such 
achromatic prisms, if perfectly achromatic, Avill treat waves of all sizes alike ; and, 
accordingly, the shortening of the waves from a moving source will not produce any 
effect. Achromatic prisms will behave to terrestrial and to extra-terrestrial sources, 
i.e., to relatively stationary and relatively moving sources, in the same way. 
Arago used an achromatic prism on a star when he showed that refractive index 
was unaffected by motion of the earth.In criticising Arago’s experiment adversely, 
Mascart forgets this, and thinks he ought to have perceived a Doppler effect. 
Mascart used a terrestrial source and an ordinary dispersive prism, when he expe¬ 
rienced the same negative result. Maxwell sent light both ways through his prism, 
* Babinet, ‘ Comptes Reiidus,’ vol. ‘J, p. 771 (1839). Akago, ‘Ann. de Chim. et de Plijs.’(^), 
vol. 37, p. 180 (1853). Maxwell, ‘Phil. Ti-ans.’ (1868), vol. 158, p. 532; also ‘Ency. Brit.’ article 
“Ether.” Hoek, ‘Archives Neerlandaises ’ (1869), vol. 4, p. 443. ^^Iascakt, ‘ Auualcs de 1 Ecole 
Norniale,’ vols. 1 and 3 (1872 and 1874) ; Professor Mascart hei'e describes a large number of negative 
experiments which he has made as to the effect of motion on most of the phenomena of optics. 
