208 



SCIENCE. 



[N. S. Vol. VIII. No. 190. 



paratus lie in the galvanometer device and 

 in the elimination of tlie effects of gravity 

 pressure in the bridge itself, as well perhaps 

 as in the self-induction and capacity ele- 

 ments. 



William Hallock. 

 Fayerweather Hall, 

 Columbia University, 

 July, 1898. 



THE ECHELON SPECTROSCOPE. 



The invention of the Echelon spectro- 

 scope is the most important advance in 

 optical research which has been made in 

 many years. It has always been the am- 

 bition of the physicist to spread the spec- 

 trum with which he was working out to 

 the greatest possible length to see if per- 

 chance the changes in color are continuous 

 or not. Thus it happened that a single 

 prism gave way to trains of prisms, each 

 member of the train doing its share to 

 wrest asunder the inconceivably minute 

 vibrations which were the objects of study. 

 With this same end in view prisms gave 

 way to diffraction gratings, these latter 

 giving purer and more diffused spectra. 

 Great labor and skill have been expended 

 in improving these gratings until now their 

 perfection has been pushed to well-nigh the 

 limit of human possibility by the masterful 

 work of Professor H. A. Eowland. 



It, therefore, becomes evident that if this 

 resolution of the spectrum is to be carried 

 notably further some entirely new means 

 of attaining the end sought must be 

 adopted. 



The important discovery by Zeeman last 

 year that the magnetic field alters the 

 nature of the vibrations of light roused 

 physicists generally to a new effort to ren- 

 der visible these changes which are very 

 small, their details lying beyond the reach 

 of the best of Professor Eowland's gratings. 



The result of these efforts is the Echelon 

 spectroscope, an instrument beautiful in 'ts 



simplicity, yet powerful beyond anything 

 ever seen before. To its inventor. Professor 

 A. A. Michelson, belongs the credit of open- 

 ing the door to a new and hitherto unattain- 

 able field of spectroscopic observation, a 

 field which promises to yield much valuable 

 information to mankind regarding the inter- 

 action between the infinitely small particles 

 of matter and those unseeu forces, elec- 

 tricity and magnetism. 



What, then, is the principle of this new 

 instrument? The theory of difiraction 

 gratings tells us that their dispersive power 

 depends on two things : First, on the dis- 

 tance between two consecutive openings ; 

 and, second, on the order of the spectrum. 

 The dispersion is greater the smaller the 

 distance between the openings and the 

 higher the order of the spectrum. The 

 efforts of physicists have hitherto been di- 

 rected towards making the distance between 

 the openings of a grating as small as pos- 

 sible as a means of spreading the spectrum 

 out as far as possible. No attempt has, so 

 far as I know, been made before this to 

 produce spectra of higher order than the 

 fourth. 



This is the point of the new instrument. 

 It is capable of giving spectra of almost any 

 order, the limit depending on the accuracy 

 with which the plates composing it can be 

 prepared. In the actual trial instrument 

 made in this laboratory the order of the 

 spectrum given is in the neighborhood of 

 20,000. 



For the mathematical theory of the in- 

 strument the reader is referred to Professor 

 Michelson's two articles on the subject.* 



One can gain a practical idea of its way 

 of working as follows : A diffraction gra- 

 ting consists essentially, as is well known, 

 of a series of equidistant openings. The 

 spectrum of the first order is formed when 

 the light in going from its source to the 



"^American Journal of Science, March, 1898 ; Astro- 

 physical Journal, June, 1898. 



