in Prof. Rowland's Concave Gratings. 199 



it is immediately seen by the figure, that the angle which is 

 formed by the straight lines AB and EF becomes equal to 



the angle S between the radii CO and CjO. AB being = 

 150 millim. the values of the differences between AE and BF 

 in the two gratings are found to be 1*08 and 1*32 millim. 

 respectively. 



In this way the asymmetry of the gratings is explained in 

 a manner as simple as it is complete. However, the found 

 differences being much greater than would be supposed in a 

 work of such perfection as are the gratings of Prof. Row- 

 land, we cannot exclude another hypothesis, the same that 

 M. Cornu* has made use of in order to explain the focal 

 properties of plane gratings, viz., a systematic variation in 

 the distances of the lines. Such a variation would arise from 

 an irregularity in the screw of the dividing-machine, and in 

 the plane gratings this explanation seems to be the only one 

 possible. But in the concave gratings the same fault might 

 also arise from another cause. The two sides of the gratings 

 giving spectra of different brightness, we may conclude that 

 the furrows which the point of the diamond makes in the 

 reflecting surface are not symmetrical in section. Then it is 

 easy to see, that the distances of the lines are subject to a 

 continual variation from one side of the grating to the other. 

 But without knowing all details of work in the ruling of 

 gratings, it w r ould be useless to enter more closely into this 

 hypothesis, and impossible to decide whether it is superior 

 to the preceding. 



* C. JR. lxxx. p. 615. 



