482 PROCEEDINGS OP THE AMERICAN ACADEMY 



On receiving the medals, Professor Rowland spoke as 

 follows : — 



Mr. President, and Gentlemen of the Academy : — 



I thank you for the honor you have conferred upon me, wliich lean 

 but regard as the greatest honor of my life. In receiving these medals, 

 I am pleased to think that they have been conferred upon work which 

 is not the result of a happy accident, but of long and persistent en- 

 deavor. 



There are some investigators whose disposition permits them to fol- 

 low their aim, inspired by the mere love of the labor and the work. 

 There are others to whom the sunshine of appreciation is necessary. 

 To either class, appreciation, when it comes, is always acceptable; and 

 I assure you that the judgment set upon my investigations by this 

 Academy is highly valued by me. 



It has been intimated that a short account of my work would be of 

 interest to the members of tlie Academy. My attention was first called 

 to the construction of dividing-engines by an inspection of a dividing- 

 engine constructed by Professor W. A. Rogers, at Waltham, in this 

 State. On returning to Baltimore, I devoted much time to the general 

 problem of such machines ; and, through the liberality of the trustees 

 of the Johns Hopkins University, I was enabled to construct an engine. 

 In about a year this engine was finished. It worked perfectly the 

 moment it was put together, and it has not been touched since. In 

 order to rule diffraction-gratings, I reflected that it was necessary that 

 the screw should be perfect, and that the rests for the plate which re- 

 ceives the ruling should also be as perfectly adjusted as is necessary in 

 optical experiments. 



The process of making the screw consisted in grinding it in a long 

 nut in which it was constantly reversed. When this screw was finished, 

 there was not an error of half a wave-length, although the screw was 

 nine inches long. 



When the dividing-engine was completed, my mind was occupied 

 with the problem of the best form of surface to receive the ruling. I 

 speedily discovered, that, by ruling the lines on a concave mirror of 

 long focus, I could dispense with a collimator and with the ordinary 

 arrangement of lenses. I now rule gratings six inches long, with 

 various numbers of lines to the inch. I find that there is no especial 

 advantage in bavins: more than fourteen thousand to the inch, with the 

 ordinary conditions of ruling. Having made the concave grating, I 

 invented a simple arrangement for mounting it, so that a photographic 



