Moon, and Starlight, and Electric Light. 9 



distinctly, this instrument affords a certainty within ten seconds, 

 and for the orange rays within half a minute. Since the whole 

 refraction through the prism amounts to about 26°, a difference, 

 which in the whole refraction amounts to y^^^, could be per- 

 ceived with this instrument ; and this would not amount to 

 the fourth part of a second in the horizontal refraction of the 

 atmosphere. It is well known that some astronomers have 

 hitherto doubted whether the tables of refractions should not 

 be, different for different stars. By the above mentioned ex- 

 periments this doubt would seem to be resolved. The con- 

 tinuation of them will, I hope, lead us to complete certainty 

 on this subject. 



In order to observe the Ji^ced lines of the different stars with 

 this large instrument, the atmosphere must be very favour- 

 able, which is rarely the case. 



The spectra from the light of Mars, and from that of Ve- 

 nus, contain the same fixed lines as the spectrum ' from sun- 

 light, and precisely in the same place, at least as far as relates 

 to the lines D, E, b, and F, * of which the relative situation 

 could be precisely determined. In the spectrum from the 

 light of Sirius, I was not able to perceive fixed lines in the 

 orange and in the yellow colours. In the green, on the con- 

 trary, a very strong streak is perceived, and two others very 

 strong in the blue. They do not appear to resemble any of the 

 lines from planetary light. 



We have ascertained their place with the micrometer. Cas- 

 tor gives a spectrum which resembles that of Sirius. The 

 streak in the green has, notwithstanding the weak light, so 

 much intensity that I could measure it, and I found it ex- 

 actly in the same place as in the green of the spectrum of Si- 

 rius. 



The streaks in the blue I could indeed distinguish, but the 

 light was not sufficiently strong to ascertain their place. In 

 the spectrum of Pollux I recognized many weak, but fixed 

 lines, which looked like those of Venus. I saw the line D 



* The line b lies in the green between E and F. It is properly com- 

 posed of three strong lines, two of which are nearer to each other than 

 the third. — Vide my '' Treatise on the Fower of Befraction and Separation 

 of Colours," S^c. p. 280 and 309. 



