OF ARTS AND SCIENCES. 477 



portion of the spectrum, while the deep notch represents the reversed 

 portion. This series of exposures was taken on a fairly clear day, but 

 occasionally wave-length 1.38 has been reached in from two to ten 

 minutes, as is shown by the two crosses on the left. 



There was a great difTerence in the transparency of the atmosphere 

 noted on different days. This was noticed by Abney. Strangely 

 enough, quite as good results have been obtained in December as in 

 May and June. 



The A line is one of the easiest in the spectrum to photograph, and 

 with the slit 5' in breadth, it may be taken in one half-second. If the 

 slit is 1' 20" wide, the spectrum may any day be photographed as far 

 as wave-length 1.00 in two minutes, and under favorable circum- 

 stances as far as 1.38, but beyond that it is not easy to obtain 

 satisfactory results. 



