and on some Phenomena connected therewith. 



201 



magnifying-power was sufficient. This lens was moved 

 gradually back until something distinctive was seen ; and the 

 distance between the lens and the grating was then measured 

 and recorded. In order to render the light more nearly mo- 

 nochromatic, pieces of red or green glass were usually held 

 in front of the eye. 



With red light the nearly equal bright and dark bars are 

 seen in focus when the distance of the lens from the grating- 

 is If inch. As the distance is increased, the definition dete- 

 riorates, and is worst at a distance of 3f . In this position the 

 proper period ('0104 inch) is lost, but subordinate fluctuations 

 of brightness in shorter periods prevent the formation of a 

 thoroughly flat field of view. As the distance is further in- 

 creased, the definition appears to improve, until at distances 

 5f and 6 J it is nearly as good as at first. The definition in 

 an intermediate position such as 6J is distinctly inferior, but 

 is far from being lost as in position 3|. From the theoretical 

 point of view, to be presently explained, these two positions 

 of extra good definition are not to be distinguished. They 

 relate rather to the sharpness of the edge of the band, than to 

 any special prominence of the proper period. At a distance 

 of 1\ we have again a place of worst definition, at 10^ a re- 

 vival, and so on. These alternations could be traced to a 

 distance of nine feet behind the grating. 



The accompanying table gives the positions of best and 

 worst definitions for red and green light respectively. Of 

 these the places of worst definition could be observed with the 

 greater accuracy ; but none of the observations have any pre- 

 tensions to precision. The star indicates the position for 

 focus. 



Eecl light. 



G-reen 



light. 



Best. 



Worst. 



Best. 



Worst. 



*lf 



H 



*li 



H 



5f,6t 



7i 



6* 



8f 



10i 



m 



11 



Ui 



151 



17 



m 



19| 



19| 



211 



22f 



25i 



24|- 



27 



27| 



31i 



29^ 



32£ 



3Si 



37 



34| 



37i 







It is evident that the positions for red light gradually fall 

 quite away from the corresponding positions for green light. 



