402 Intelligence and Miscellaneous Articles. 



as to the substances to which they belong. The line b of magnesium 

 is also very good, especially for coloured stars. These lines ought 

 then to have the same place in the spectra of all the stars as regards 

 their direct image, if they are not displaced by the motion. Let us 

 now come to the results. 



The apparatus I have used consists of a spectroscope for direct 

 vision, in which for the slit a cylindrical achromatic lens is substi- 

 tuted. The prism is arranged so that part of the rays forming the 

 luminous line in the focus of the cylindrical lens traverse it and are 

 dispersed, while a small portion is transmitted directly beyond the 

 prism. We thus have in the field of the analyzing telescope two 

 images — one dispersed with the rays well defined, the other white 

 and in line. If the prism is suitably cut, we may have exact coin- 

 cidence between a dark line and a direct white line, by slight varia- 

 tions in the inclination of the axis of the prism. If this coincidence 

 cannot be obtained (which is the case in certain prisms), it is ne- 

 cessary to place in the eyepiece two micrometric wires, and the dis- 

 tance of the line from the white image is measured as in the case 

 of double stars. 



The simple spectroscope may also be used with great advantage, 

 for the light is very bright ; but if the prism has great dispersive 

 power, the divergence which it produces in the rays traversing it 

 must be compensated by passing the direct pencil through a concave 

 semilens of such a focus as to have the lines and the image of the 

 star in the same focus. We might dispense with the cylindrical 

 lens ; but though it is possible to see the rays, they have never the 

 definition and precision which the lens gives. With the simple spec- 

 troscope it is still more easy to make the direct image coincide with 

 any line of the spectrum. Although the apparatus was constructed 

 with the pieces I had at hand, I have found that we can very readily 

 recognize the displacement of the line F, if it is not by an amount 

 greater than its own magnitude — that is, a limit almost equal to that 

 above mentioned ; for the line F is not twice as great as the double 

 line D. 



With these two kinds of apparatus I have analyzed a great part 

 of the stars up to the third magnitude which are now to be seen in 

 the evening. In each series of observations the luminous band of 

 Sirius was first made to coincide with its line F, so that the white 

 line was exactly divided by a very acute point placed in the field of 

 the eyepiece. This opake point, since it almost completely conceals 

 the white line, facilitates the observation of those spectral lines whose 

 light would be enfeebled by its brilliancy. This result being attained 

 without touching the eyepiece, I directed the telescope to another 

 star to ascertain whether a similar coincidence existed. 



After having frequently repeated these comparisons, I arrived at 

 the conclusion that in the case of stars belonging to the type of Sirius 

 there is no displacement which is appreciable by my means of measure- 

 ment. In the case of stars of a different type, like a of Orion, 

 I made use of other lines, and particularly the line b of magnesium, 

 which is very well defined, after having confirmed the coincidence of 



