THE 



AMERICAN J0URNAL0FSC1ENCE 



[FOURTH SERIES.] 



Art. X. — On an Adjustment for the Plane Grating Simi- 

 lar to Rowland's Method for the Concave Grating /* by 

 Carl Barus and Maxwell Barus. 



1. Apparatus. — The remarkable refinement which has been 

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

 celluloid replicas of the plane grating, makes it desirable to 

 construct 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 inexpensive contrivance, in which the wave-length 

 is strictly proportional 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 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 tnost of the usual errors 

 should be eliminated by symmetry. It is also shown that the 

 symmetrical method may be adapted to the concave grating. 



In fig. 1, A and B are two double slides, like a lathe bed, 

 155 cm long and ll om 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 Rowland's, would have been preferable. The car- 

 riages and D, 30 cm long, kept at a fixed distance apart by 

 the rod aRb, are in practice a length of ^-inch gas pipe, 



* The greater part of this work was done by my son and myself and con- 

 tributed to the Proceedings of the American Philosophical Society, April 

 24, 1909, from which the present paper is abridged. I have since added 

 some other matter at the end. — C. B. 



Am. Jour. Sci. — Fourth Series, Vol. XXXI, No. 182. — February, 1911. 



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