1880-1 on the Photographic Spccfm of the Slavs. 289 



iiuiige being rather larger than the width of the slit, its place, even 

 when upon the slit, can be seen. If, therefore, tlie observer keeps his 

 eye fixed upon tlie star's image during; the wliole time of exposure, half 

 an hour, one hour, or it may be two hours, he can instantly correct by 

 hand any small irregularities of the motion of the telescope, and so 

 maintain the star's image invariably fixed upon the slit. 



Further, it was necessary to obtain the photographs under such 

 conditions that it should be possible afterwards to determine with 

 accuracy the value in wave lengths of the positions in the spectrum of 

 the stellar lines. 



For this purpose the slit was provided with two small shutters, as 

 represented at h and g, Fig. 1. One of these only remains open 

 while the photograph of the star is taken. 



When the exposure is finished this shutter is closed. The other 

 can then be opened, and a second spectrum upon the same plate for 

 the purpose of comparison taken. It may be the solar , spectrum 

 reflected from the moon, or the spectrum of a known star, or a 

 terrestrial spectrum, or the apparatus may remain until the following 

 day, and then the solar spectrum be taken U2)on the plate directly. 



Afterwards, from these comparison spectra, by the aid of a suitable 

 measuring apparatus attached to a microscope, the wave lengths of the 

 stellar lines were determined. And for this purpose use was made 

 of the excellent map of M. Corn'u of the ultra-violet, and of his 

 determinations, and those of Mascart, of the wave lengths of the lines of 

 cadmium, aluminium, and zinc. Various, photographic methods were 

 tried, but the great sensitiveness which may be given to gelatine 

 plates, as well as the great advantage of employing plates in a dry 

 state, led to the exclusive use of this method of photography. 



I was about to complain of how few nights sufficiently fine for 

 this work present themselves during a whole year — they may be 

 counted upon the fingers — but I forbear when I remember that, not- 

 withstanding the terrible drawbacks of our climate, no country con- 

 tributes more largely than our own to the advance of astronomy. 



Before proceeding to the results of my work, I will endeavour to 

 make visible to you some portion of the ultra-violet part of the 

 spectrum. 



Besides their photographic power, there is another mode of action 

 by which the ultra-violet rays may make themselves visible to us. 

 There are some substances which absorb these very rapid vibrations, 

 and then give back the energy they liave received, in the form of 

 vibrations which are sufficiently long to come within the power of the 

 eye. They transform the invisible energy into visible light. This 

 property of fluorescence is possessed in a high degree by sulphate of 

 quinine, and by aesculin, a substance which exists in the bark of the 

 horse-chestnut. I have a small screen which has been brushed over 

 with a solution of this substance. 



Professor Dewar has kindly placed at my disposal one of his 

 electric-arc crucibles. I cannot forbear congratulating Professor 



Vol. IX. (No. 72.) x 



