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



(2) The brightness of the superposed spectrum increases pro- 

 portionally to the breadth of the slit. 



(3) With an oscillating or rotating slit the brightness of the 

 superposed spectrum remains unaltered ; that of the image of 

 the protuberance decreases according to a law which depends 

 upon the number and duration of the impressions produced on 

 the place of the retina in question in the unit of time, and on 

 the refrangibility of the observed protuberance-band. 



Assuming, for simplicity's sake, that the entire surface over which 

 the slit moved in its rotation or oscillation were occupied by the 

 protuberance, and assuming that the intensity of the after-image 

 formed were inversely proportional to that surface (corresponding 

 to a uniform distribution over that surface of the light passing 

 through the stationary slit), then assuming the above three prin- 

 ciples, the ratio of the intensity between ground and protube- 

 rance would remain the same, whether, 



First, by oscillation of the slit the brightness of the image 

 of the protuberance were diminished, and thus the brightness 

 of the superposed spectrum or of the ground (according to (2)) 

 were left unchanged, or whether, 



Secondly, the stationary slit was so far opened that its aper- 

 ture just extended over the space over which in the first case the 

 oscillation extended. Hereby, according to (1), the apparent 

 brightness of the protuberance would remain unchanged, while 

 that of the ground would be increased in the same ratio in which 

 it was formerly weakened when the ground was unaltered. 



Hence, under the above suppositions, the intended object would 

 be far more simply attained in the second way, by taking care 

 that, on account of dazzling, the intense direct light of the sun 

 did not penetrate into the slit. 



The slit need then only be opened so far that the protuberance, 

 or a part of it, appears in the aperture. By polarizing or absorb- 

 ing media, placed in front of the eyepiece, a suitable weakening 

 of the entire field of view must be provided for, in order that 

 the ratio between the intensities of the protuberance and su- 

 perposed spectrum may be as striking as possible. 



Led by these considerations, I have attempted to realize by 

 means of terrestrial sources of light the conditions under which 

 the protuberances are visible, in order thus to test both methods 

 and convince myself of their practicability. In order the better 

 to understand the experiments described, the following remarks 

 may be premised. 



The reason why, under ordinary circumstances, by deadening 

 the intense solar image the protuberances are not visible at its 

 edge, lies in the superposed strongly illuminated particles of our 

 atmosphere. In a total solar eclipse this superposed light is so 



