J. M. Blake—Notes on Diffraction Gratings. 35 
not produced a radical change in the accuracy of position of 
the lines. As proof of this may be mentioned the constancy 
of obtaining certain results now to be described. The follow- 
ing appearances were produced without any visible connection 
with the accidental bands on the photographs themselves, 
- provided they were not viewed by light more oblique than 
only second secon spectrum observed by lamp-light was: 
ting. 
The bands develo by superposition were photographed. 
This was first a ieeeal by diffused light, and the best results, 
were obtained when the light was oblique. But it was found. 
