361 M. F. Zollner on a New Spectroscope, together with 



scribed whether the expected displacement of spectral lines occurs 

 or not. In reference to the requisite intensity of light, I may 

 be permitted to remark that I had a non-achromatic lens* of 

 1 Paris foot diameter and 6 feet focal distance ; the pencil was 

 received a few inches in front of its focus on a suitable concave 

 meniscus of flint glass, and, thus freed as far as possible from 

 spherical and chromatic aberration, it impinged on the slit of the 

 spectroscope. I think I must here more especially point out that, 

 in the use of a slit, achromatism of the optical image is not ne- 

 cessary for the observance of the spectrum, especially of indivi- 

 dual parts of it, and that therefore the above construction may 

 claim the advantage of being cheaper than when achromatic 

 glasses of great luminous intensity have to be used. Of course 

 in those cases in which the objects to be observed require as 

 sharp separation as possible, as in the case of the double stars, 

 this advantage must be given up. 



I may be permitted, in conclusion, to make a few observations 

 on problems and methods which refer to spectrum-observations 

 of the sun, and with which I am at present occupied. 



The sun possesses a velocity of rotation in virtue of which a 

 point on its equator moves with a velocity of about a mile in a 

 second. If, therefore, by means of a heliometer, or in any other 

 way, a double image of the sun be produced, and if by suitable 

 adjustment two points of the edge of the equator be brought into 

 contact, parts of the sun's surface are bounded by the point of 

 contact, of which one set move towards us and the other move 

 from us with a velocity of the amount mentioned. There is 

 thus produced a difference in the velocity of the parts touching 

 of about two and a half miles. In accordance with what has 

 been above said, such a magnitude of motion would produce an 

 alteration in the position of the sodium-lines corresponding to 

 the ( sVh part of their distance. Hence if, by combining a suffi- 

 cient number of prisms, such a magnitude can be perceived or 

 measured, it is only necessary to bring the middle of the slit to 

 the line of the two centres of the sun's pictures to see in the 

 field of view of the spectroscope the two spectra of the sun's 

 edges close to one another, and thus observe the displacement in 

 question under the most favourable circumstances. In this 

 manner the position of the sun's equator might be determined ; 

 and, provided the measurements could be executed,, the velocity 

 of rotation in various heliographic latitudes might also be deter- 

 mined, which would be of the greatest interest in reference to 

 opinions recently expressed upon this subject. 



Apart, however, from a quantitative determination of the phe- 

 nomenon in question, by even a qualitative proof a simple means 



* Constructed iu the optical workshop of M. II. Schroder in Hamburg. 



