70 Is Light Imponderable? 



ing the instrument from place to place. The difficulties of 

 making such a spectroscope were very great ; but they have 

 been surmounted with marvellous dexterity and skill. A full 

 description of this instrument will be found in the Proceedings 

 of the Royal Society, No. 76. 



It consists of two prisms of dense flint glass, made by 

 Chance, and having a specific gravity of 8 - 9. These prisms are 

 mounted on a bed of slate. Light reaches them through a 

 slit, and straight through a 'portion of a right-angled prism, 

 where the refraction is neutralized by a small prism cemented on 

 to it. After passing through the two large prisms, and once 

 suffering refraction and dispersion, it is sent back through 

 them a second time by a third prism, having one side silvered, 

 and thus it is again refracted and dispersed, one set of prisms 

 being compelled to do double work. On its return journey, 

 the light enters the prism over the slit and is reflected 

 to a telescope at right angles to it, and carrying a micro- 

 meter. A series of trials in Mr. Browning's workshop, 

 at Kew, and in the apartments of the Eoyal Society, have 

 shown that the variation of the D line is quite infinitesimal, in 

 spite of great changes of temperature and motion from place 

 to place. 



It was at first intended to send a " rigid spectroscope " up 

 in a balloon; but the weight of the present form, and the un- 

 certainty in balloon experiments of the prisms acquiring a 

 uniform temperature, caused this idea to be abandoned, and 

 Mr. Gassiot invites the Eoyal Society to request Her Majesty's 

 Government to send the instrument on board some ship sailing 

 between points on the earth's surface differing considerably in 

 latitude, and, consequently, in the force of terrestrial gravita- 

 tion. Should such experiments succeed, light must no longer 

 be considered as the motion of an imponderable substance. 

 Should they fail, it may still be held that success would be 

 possible, if stations much nearer to, and much remoter from, 

 the earth's centre, could be reached. 



We must not be understood as stating that the contemplated 

 experiments are attempts to weigh light; but if it can be 

 proved that any change in the direction and force of gravita- 

 tion affects the position of any line in the rigid spectroscope, 

 the ponderability of that fluid of which light is a mode of 

 motion might be probably assumed. Is gravitation merely a 

 mode of force, correlative with other forces known and un- 

 known ? 



