ON AN ADJUSTMENT FOR THE PLANE GRATING 



SIMILAR TO ROWLAND'S METHOD FOR 



THE CONCAVE GRATING.^ 



By carl BARUS. 

 (Read April 24, 1909.) 



I. Apparatus. — The remarkable refinement which has been at- 

 tained (notably by Mr. Ives and others) in the construction of 

 celluloid replicas of the plane grating, makes it desirable to con- 

 struct a simple apparatus whereby the spectrum may be shown 

 and the measurement of wave-length made, in a way that does 

 justice to the astonishing performance of the grating. We have, 

 therefore, thought it not superfluous to devise the following inex- 

 pensive contrivance, in which the wave-length is strictly propor- 

 tional to the shift of the carriage at the eye-piece; which for the 

 case of a good 2-meter scale divided into centimeters, admits of a 



o 



measurement of wave-length to a few Angstrom units and with 

 a millimeter scale should go much further. 



Observations are throughout made on both sides of the incident 

 rays and from the mean result most of the usual errors should be 

 eliminated by symmetry. 



In Fig. I, A and B are two double slides, like a lathe bed, 155 

 cm. long and 11 cm. apart, which happened to be available for 

 optical purposes, in the Laboratory. They were therefore used, 

 although single slides at right angles to each other, similar to Row- 

 land's, would have been preferable. The carriages C and D, 30 cm. 

 long, kept at a fixed distance apart by the rod aRh, are in practice 

 a length of ^-inch gas pipe, swivelled at a and b, 169.4 centimeters 

 apart, and capable of sliding right and left and to and fro, normally 

 to each other. 



^ The investigations in this paper were undertaken throughout in con- 

 junction with my son, Mr. Maxwell Barus ; but it seemed advisable that I 

 should undertake the publication in these Proceedings myself, with the 

 present acknowledgment. 



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