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XV. On a Spectrum- Telescope. By Dr. P. Glan*. 



SINCE the discovery of methods for examining the solar 

 prominences at all times, and since hitherto such exami- 

 nation has been confined to only small parts of the circum- 

 ference at the same time, the efforts of physicists and astrono- 

 mers have been directed to the simultaneous observation of 

 .arger portions of the sun's edge in monochromatic light. 

 These efforts have become of more importance since the ob- 

 servation was made that the passage of a star behind the sun's 

 limb takes place at different times, according as the sun is 

 observed in the usual way (in white light) or in spectral 

 homogeneous light. The latter method of experimenting, as is 

 well known, consists in throwing an image of the sun from 

 the objective of a telescope upon the slit of the spectroscope, 

 and examining through the telescope of the latter a small part 

 of the spectrum of that portion of the solar image which falls 

 upon the slit. Even when the dispersion is great and the slit 

 as wide as possible, we cannot by this means see more than a 

 small part of the sun at one time. I shall describe here a 

 method of observation which allows a considerably larger part 

 of the sun to be seen at once in homogeneous light, and which 

 permits the telescope to be used in the ordinary way, namely 

 with its eyepiece and cross-wires — so that, for instance, it is 

 easy to measure the sun's diameter as seen in the different 

 spectrum-colours. 



The method is the following: — Upon the eyepiece of a tele- 

 scope I screw a small direct-vision spectroscope with collimator 

 and telescope. In the latter, at the place where the spectrum 

 is thrown, there is a movable diaphragm with a slit-sbaped 

 opening. The spectroscope is so adjusted that the Fraun- 

 hofer lines and the edges of the diaphragm are seen clearly 

 at the same time. The eyepiece of fhe spectroscope is un- 

 screwed, and the eye is placed immediately behind the slit of 

 the diaphragm, after the slit and that of the collimator have 

 been widened to about one third of a millimetre. The eyepiece 

 of the telescope with the spectroscope screwed to it is then 

 moved till the image of the sun becomes distinctly visible. 

 The image of the sun thrown by the objective of the telescope 

 thus gives rise, by means of the eyepiece of the telescope and 

 the collimating lens of the spectroscope, to a second image, 

 which in my apparatus falls immediately in front of the train 

 of prisms. This image is seen enlarged through the objective 

 lens of the telescope (which acts as a magnifying-glass), in 



* Translated from the Astronomische Nackrichten, No. 2309, with ad- 

 ditions communicated by the Author. 



