10 ABSORPTION SPECTRA OF SOLUTIONS. 



strip, and so on. It was always found advisable to clean the slit-jaws after 

 each exposure, and also to see that the image of the Nernst filament fell in 

 the proper position on the slit. 



After the film had been exposed and the comparison spark spectrum 

 impressed, it was necessary to make a series of exposures on a panchro- 

 matic plate for the red end of the spectrum, using the same set of solutions. 

 No exposure to the spark was made in this set except for the narrow com- 

 parison strip. As the extreme red end of the plate was at about A 7600, 

 A 3800 of the second order would overlap the first order here. Accordingly 

 an absorbing screen was always used in making the exposures for the red 

 end of the spectrum. This screen consisted of two glass plates separated 

 by a layer of Canada balsam a little less than a millimeter thick. It 

 absorbed all radiations of shorter wave-length than A 3900. 



The scale accompanying the spectrograms in the following chapters 

 was made by photographing an ordinary paper scale. Several photo- 

 graphs were made, the distance between the paper scale and the lens of 

 the camera being varied slightly from exposure to exposure. The resulting 

 negative which fitted the majority of the spectrograms best was selected 

 and used throughout. Absolute accuracy is not to be expected, owing to 

 the fact that both photographic films and the paper on which prints from 

 these were made, contract more or less in drying, and different films or 

 papers contract differently. ^ 5000 on the scale was always placed in coin- 

 cidence with the corresponding wave-length on the photographic strips; 

 the correction for the ends of the spectrograms differs slightly for the 

 different plates, but never amounts to more than about 25 or 30 Ang- 

 strom units. 



