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



the right of them. Without the spectroscope, with white light 

 I saw, in the case of the larger spots, the very black nuclear 

 speck and the penumbra with admirable sharpness. With the 

 spectroscope there were also seen all the spots, and in some 

 the nuclear specks and penumbrce, although they were not so 

 sharply separated. In the methods hitherto described, when 

 the spectroscope was added it was necessary to alter the ad- 

 justment of the telescope. If it were required to make use 

 of the spectroscope and still leave the telescope totally un- 

 altered in position, as it is usually adjusted with its micro- 

 meter cross-piece, an auxiliary lens has to be interposed 

 immediately between the slit and the eye-diaphragm. With 

 my telescope I used a lens of 67*5 millims. focal length. When 

 I arranged the telescope in the ordinary way and then fastened 

 to the eyepiece the convex lens and the spectroscope, one im- 

 mediately behind the other, I was able to see the objects in 

 homogeneous light as clearly as previously in white light, 

 without having to alter the adjustment of the telescope in the 

 least. Other lenses would have to be selected for other tele- 

 scopes. 



The arrangement of the apparatus hitherto adopted, which 

 was made by Schmidt and Haensch in Berlin, was only a pro- 

 visional one. The prisms and lenses of this spectroscope were 

 selected without any special examination of their surfaces ; 

 and this may have contributed to deprive the homogeneous 

 images of their highest possible sharpness. I hope to be able 

 to have the apparatus made in a more complete form. 



Addition {Dec. 21, 1880). 



I add a few data concerning the focal length of the lenses 

 and the dimensions of the spectroscopes employed. The focal 

 length of the collimator-lens was 40 millims. ; its free opening 

 was 8 millims. The focal length of the objective lens of the 

 telescope of the spectroscope was 44 millims. ; its free opening 

 was 12 millims. The train of Amici prisms was 75 millims. 

 long. The total length of the spectroscope (without its eye- 

 piece), which was screwed on to the eyepiece of the terrestrial 

 telescope, was 1 65 millims., measured from the slit of the col- 

 limator to that of the eyepiece. 



I may add the following to the previously described obser- 

 vations on the sun-spots. The spots, when viewed in orange, 

 green, and especially in blue, homogeneous light, appeared 

 distinctly darker than in red light. The difference appeared 

 to me to be most noticeable when the air was exceptionally 

 clear, and failed as the weather clouded. As this estimation 

 depends upon judgments of the luminosity of the sequent 



