44 



a share in determining ttie 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. The construction of this 

 *' Eigid Spectroscope," therefore, was to ascertain whether the 

 position of the well-defined lines of a spectrum is constant^ 

 while the co-efficient of terrestrial gravity under which the 

 observations are taken is made to vary. 



The difficulties of making such a Spectroscope were very 

 great, but they have been overcome, and a series of trials in 

 Mr. Browning's workshop at Kew, and in the apartments of 

 the E*oyal Society, have shewn that the variation of the I> 

 line is quite infinitesimal, in spite of great changes of tem- 

 perature, and the removal of the instrument from place to 

 place.* 



Great interest has of late years been attached to the com- 

 position of Meteors, shooting Stars, and Bolides, to learn 

 something of which, Mr. Alexander Herschel has constructed 

 a direct-vision Spectroscope, fitted up with peculiar prisms for 

 binocular arrangement, having a wide field of view and great 

 power. This instrument, like the former, presented considera- 

 ble difficulties of ojDtical construction, which have been over- 

 come by Mr. Browning. 



Observers furnished with a Binocular Spectroscope may direct 

 it like an opera- glass to that portion of the sky where the 

 Meteors are expected to fall vertically towards the Earth, near 

 (but not too near) the radiant point, where their course is 

 foreshortened, and their apparent motion comparatively slow. 

 As Meteors and shooting Stars make lines of ligbt in the sky, 

 each will give one or more lines of light in the Spectroscope. 

 If, for example, we had a Sodium Meteor, it would give a 

 yellow line in the sky, and a yellow line occupying the place of 

 the Sodium line in the Spectroscope. If Silver were present, 

 together with Sodium, the Meteor train would have a greenish 

 tinge, and three lines would appear in the Spectroscope ; viz., 

 yellow, green, and hlue. The spectral examination of the 

 August Meteors has brought to light the fact of the existence 



■* Since these remarks were made on the Rigid Spectroscope, it has come to my 

 knowledge that the instrument has been put on board of H.M. Ship Nassau, in charge 

 of Captain Mayne, who was preparing to make a survey of the Straits of Magellan. A 

 lengthy correspondence with some results has been received by J. P. Gassiot, Esqr., 

 from Captain Mayne, which was placed in the hands of Pro. Stokes, and Mr. B. 

 Stewart, from which it appears that the result at present influencing the variation of 

 gravity does not exceed in passing from Lat. 45° to the Equator, a change of refraction 

 for the yellow of the spectrum equal to about three-fourths of the interval of the 

 D-lines, but more observations are required on the ship's return before it can be 

 asserted that this apparent change is not due to known causes. 



