Rowland's Method for the Concave Grating. 95 



interference, it is shown that many of the errors are elimi- 

 nated by the symmetrical adjustments in question. The slide 

 carrying the grating may he provided with a focusing lens in 

 front or again behind it, if the means are at hand for actuat- 

 ing the slit which is not sharply in focus on the plane of the 

 eye-piece carried by a second slide throughout the spectrum at 

 a given time. It is thus best to use both lenses conjointly, 

 the latter as a collimator and the former as an objective of the 

 telescope in connection with the eye-piece. It is shown that a 

 centimeter scale parallel to the eye-piece slide with a vernier 

 reading to millimeters is sufficient to measure the wavelengths 

 of light to few Angstrom units, while the wave lengths are 

 throughout strictly proportional to the displacements along 

 the scale. The errors of the three available methods and their 

 counterparts are discussed in detail. The method is applica- 

 ble both to the transparent and the reflecting grating. 



It is furthermore shown that in case of Rowland's concave 

 grating observation may be made symmetrically on both sides 

 of the slit, or for reasonable clearance of slit and eye-piece 

 passing across each other, although one conjugate focal dis- 

 tance is now not quite the projection of the other. 



Brown University, Providence, E. I. 



