Is Light Imponderable 1 ? 69 



that as they counterbalanced each, other at the equator they 

 would do so at the poles., for whatever alteration in weight 

 resulted from their change of place would affect both alike. 

 The tension exerted by a spiral spring affords, as Sir J. 

 Herschel points out, an illustration of a kind of force not 

 affected by change of position ; and such an instrument might 

 enable us to compare the attraction of the earth upon a given 

 mass in the two situations described. 



These preliminary remarks may help those who have not 

 reflected on the subject to consider weight simply as the result 

 •of attraction; and, as our instruments are all comparatively 

 clumsy, it is not right for us to assume that a particular body 

 possesses no weight at all because we have not succeeded in 

 weighing it. If light be a mode of motion of some fluid, 

 that fluid may not, as is commonly supposed, be absolutely 

 imponderable, that is, not at all affected by the earth's attrac- 

 tion. We have, indeed, no right to assume that gravitation 

 is an essential property of all matter under all conditions. 

 Astronomers trace what we ordinarily call the universality of 

 this force ; but that it is really universal — that is to say, that it 

 exists wherever matter exists — is more than we know. It is 

 common to speak of light, heat, electricity, etc., as " imponder- 

 able agents/' but we apprehend no thinking man is satisfied 

 with such a phrase. If they are all modes of motion of some 

 kind of matter, either that matter must be ponderable, or 

 gravity only a property manifested under certain conditions! 



Mr. Balfour Stewart and Professor Tait consider, in the 

 technical language employed by the former, " that to this time 

 it has been assumed, without proof, that the change in the 

 co-efficient of terrestrial gravity does not in itself alter any 

 other co-efficient of a body; and if a reason be asked none can 

 be given, since gravity is a force of the nature of which men 

 of science are confessedly ignorant." 



Now, if gravitation acts upon light so as to have any share 

 in determining the position of any rays emerging from a prism, 

 and forming a spectrum, a considerable change in the position 

 of such a prism and of such light rays, involving a change in 

 the force of gravitation, might cause a dark line in the spectrum 

 to take a new position, more or less differing from that which 

 it first assumed. 



When this notion was propounded to Mr. Gassiot, he acted 

 with his accustomed liberality in the cause of science, and 

 requested Mr. Browning to construct a " rigid spectroscope." 

 That is, such a spectroscope that the position of any given 

 line could be exactly measured with minute accuracy, and 

 with the certainty that its position would not be changed 

 by the action of heat, or by the jolting inevitable in transport- 



