470 Prof. H. A. Rowland on the Results accomplished 



shall soon publish, by which I hoped to make a practically 

 perfect screw; and so important did the problem seem, that I 

 immediately set Mr. Schneider, the instrument-maker of the 

 university, at work at one. The operation seemed so success- 

 ful that 1 immediately designed the remainder of the machine, 

 and have now had the pleasure since Christmas of trying it. 

 The screw is practically perfect, not by accident, but because 

 of the new process for making it; and I have not yet been 

 able to detect an error so great as one one-hundred-thousandth 

 part of an inch at any part. Neither has it any appreciable 

 periodic error. By means of this machine I have been able 

 to make gratings with 43,000 lines to the inch, and have 

 made a ruled surface with 160,000 lines on it, having about 

 29,000 lines to the inch. The capacity of the machine is to 

 rule a surface 6£ x 4£ inches with any required number of 

 lines to the inch, the number only being limited by the wear 

 of the diamond. The machine can be set to almost any num- 

 ber of lines to the inch; but I have not hitherto attempted 

 more than 43,000 lines to the inch. It ruled so perfectly at 

 this figure that I see no reason to doubt that at least two or 

 three times that number might be ruled in one inch, though 

 it would be useless for making gratings. 



All gratings hitherto made have been ruled on flat surfaces. 

 Such gratings require a pair of telescopes for viewing the 

 spectrum. These telescopes interfere with many experiments, 

 absorbing the extremities of the spectrum strongly ; besides, 

 two telescopes of sufficient size to use with six-inch gratings 

 would be very expensive and clumsy affairs. In thinking 

 over what would happen were the grating ruled on a surface 

 not flat, I thought of a new method of attacking the problem ; 

 and soon found that if the lines were ruled on a spherical sur- 

 face, the spectrum would be brought to a focus without any 

 telescope. This discovery of concave gratings is important 

 for many physical investigations, such as the photographing 

 of the spectrum both in the ultra-violet and the ultra-red, the 

 determination of the heating-effect of the different rays, and 

 the determination of the relative wave-lengths of the lines of 

 the spectrum. Furthermore it reduces the spectroscope to 

 its simplest proportions, so that spectroscopes of the highest 

 power may be made at a cost which can place them in the 

 hands of all observers. With one of my new concave gratings 

 I have been able to detect double lines in the spectrum which 

 were never before seen. 



The laws of the concave grating are very beautiful on 

 account of their simplicity, especially in the case where it 

 will be used most* Draw the radius of curvature of the mirror 



