igi^.] 



RICHARDS— AUTOCOLLIMATING MOUNTING. 



555 



mounted the slit on an arm pivoted at the center of the line join- 

 ing them (Fig. 2). These methods however require the source of 

 light to be movable, which is usually undesirable and in some cases 

 impracticable. Wadsworth* suggested several arrangements to over- 

 come the difficulty, using auxiliary mirrors and more or less com- 

 plicated mechanism, but these involve additional adjustments and 

 loss of light. It may be added that the grating has also been used 



Fig. I. 



with parallel light in astronomical work, but the aberration is much 

 greater than with Rowland's mounting.^ 



The method here discussed is briefly that of autocollimation. 

 That part of the light is used which after being dififracted is re- 

 turned toward the slit. If therefore the slit is on Rowland's circle, 

 the spectrum will be formed on the same circle and one point of it 

 will coincide with the slit (Fig. 3). The ingoing and outcoming 

 beams may be separated when necessary by the usual reflecting 

 prism, or by slightly tilting the grating. Thus a double slit may be 



*F. L. O. Wadsworth, Astrophys. Jour., 2, 370 (1895). 

 *F. L. O. Wadsworth, Phil. Mag. (6), 6, 119 (1903). 



