43 



of ttis nature very difficult. On but few of those nights when 

 the stars shine brilliantly is the air sufficiently steady for such 

 very delicate observations. The light of the Stars is also very 

 feeble, and requires a large instrument with an object glass of 

 not less than eight inches aperture to gather up and concen- 

 trate to a focus the light from so small an object. 



Another inconvenience arises from the apparent motion of 

 the Stars caused by the rotation of the Earth. This difficulty 

 has been overcome by the application of clock work, by which 

 the telescope and observer are moved in a direction opposite 

 to that of the Earth, by which means the object is kept within 

 the focus of the instrument. In practice, however, it is found 

 not easy to retain the image of a Star steady for any length of 

 time exactly within a slit, the jaws of which are only l-300th 

 of an inch apart. 



These difficulties being overcome, the light produced by a 

 terrestrial substance is compared with the Stellar Spectrum by 

 means of a prism fixed over one half of the slit, which receives 

 the light reflected into it by a movable mirror that faces a 

 clamp of ebonite, provided with forceps to hold the fragments 

 of metals employed. These metals are rendered luminous in 

 the state of gas, by the intense heat of sparks produced from a 

 powerful induction coil. The light from these sparks is 

 reflected into the instrument by means of the mirror, and a 

 small prism being placed in connection with the larger one, 

 which receives the light from the Star, the two spectra are 

 visible in juxtaposition, by means of a small telescope ; and 

 thus the coincidence and relative position of the bright lines 

 in the spectrum of the Star can be accurately determined by 

 the bright lines in the metallic sj)ectrum. 



The spectroscope, as an instrument of research applied to 

 scientific discovery, had never a more fitting subject to operate 

 upon than that for the investigation of which the " Rigid 

 Spectroscope" was contrived by J. P. Gassiot, Y.P.E.S., and 

 manufactured by Mr. Browning. 



It is customary to speak of light, heat, electricity, &c., as 

 " imponderable agents," or as gravity, a property manifested 

 under certain conditions, Mr. Balfour Stewart, and Pro- 

 fessor Tait, of Edinburgh, in technical language employed by 

 the former, state : " 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 nature, of which men of science are confessedly 

 ignorant." Now, if gravitation acts upon light so as to have 



