Dr. P. Glan on a Spectrum- Telescope. Ill 



that colour of the spectrum of the collimator's slit which, fall- 

 ing upon the diaphragm of the eyepiece, passes through its 

 slit. I used a terrestrial telescope of Schmidt and Haensch 

 of 50 millims. aperture ; and by this means I was enabled to 

 see nearly half the sun at once in homogeneous light. Seen 

 by the naked eye, the sun appeared white in almost all parts 

 of the spectrum excepting in the red, where it assumed a yel- 

 lowish-white tint. This agrees with the observation that all 

 colours become white if of sufficient intensity. The image at 

 parts of the edge at right angles to the slit, to a distance of 

 about 40° on both sides, was perfectly sharply defined. The 

 other parts of the sun's edge were less sharp. These slight 

 residues of lateral diffraction are probably chiefly due to the 

 circumstance that every point of the sun's image gives a line- 

 spectrum through the train of prisms, and that the slit-shaped 

 opening before the eye allows a little line of this spectrum, 

 and not a mere point, to pass through. 



This residual diffraction, however, is only slight; and I hope 

 to eliminate it further by future improvements in the apparatus. 

 Further, it is possible to see every part of the sun's edge per- 

 fectly distinctly. For this it is only necessary so to turn the 

 spectroscope that its slit stands at right angles to the edge 

 under observation. Such an attitude of the slit would, for 

 instance, be advantageous in observing the beginning of the 

 transit of a planet. I have found that any diffraction which 

 might be caused by the slits of the spectroscope is sufficiently 

 removed by giving the slits a width of ^ millim., or at most 

 -§• inillim. If the slit be narrower than ^ millim. the edges 

 parallel to the slit appear very washed out ; and when the slit 

 is exceedingly narrow, the sun's image is pulled completely 

 out broadwise. A slight diffraction may also have been caused 

 by the use of a train of direct- vision prisms, which does not 

 refract homocentrically the sun's image, which is in this case 

 at a finite distance from it. I would therefore, in future, re- 

 commend the use of an equilateral prism, employing the angle 

 of minimum refraction : this is always homocentric. 



In the examinations of the sun which I undertook, from the 

 29th of January to the 10th of February, 1880, I was unable 

 to detect any protuberant eminences on the sun's edge. This 

 was no more than was to be expected, from the slight disper- 

 sion of the train of prisms. On the other hand, I have been 

 able to observe satisfactorily the beginning and changes of 

 sun-spots. I shall describe one observation. On the 4th of 

 February I saw a large spot close to the upper edge, a second 

 one of about the same diameter more towards the centre, and a 

 group of three (two larger, and a smaller weak nebulous spot) to 



