106 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Spark line structure as a function of the inductance. 



78 (c). 4810. Between single and double order condition. Center 

 of 1.5 mm. gap. Three layers of inductance coil (a). Exposure: 

 1 minute. Two main components. Note that the right component 

 of the quadruplet is as strong as, or stronger than the left, when the 

 position of orders is such that it would be weaker. This fact is con- 

 firmed by other exposures. 



79 (e). 4722. Between single and double order condition. Elec- 

 tric conditions as in 78 (c). Exposure: 1 minute. Shows three main 

 components. 



80 (a). 4680. Between single and double order condition. The 

 electric conditions are as in 78(c) and 79(e). Two main components. 

 Exposure: 1 minute. 



65 (b). 4810. Single order condition. Center of 4 mm. spark gap 

 under different conditions. Outside, no inductance, 5 seconds: 

 inside coil (b) in circuit, 45 seconds. Notice the two side components 

 in the inductance spark image. 



68 (a). 4722. Single order condition. Electric conditions, simi- 

 lar to 65. Note inequality of intensity of inductance line components. 

 Exposures: 30 seconds with inductance and 3 seconds without. 



94 (c), (d), and (e). 4810. Single order condition. Center of a 

 very small gap — ■ less than 2 mm. Three, two and one layers of 

 coil (b), respectively. 



96 (c), (d), and (e). 4680. Double order condition. Same set 

 of operations as in 94. Notice in both plates a continuous increase 

 of intensity of the old components lying toward the red and the 

 development of new ones as the inductance is decreased. Another 

 photographic plate (numbered 95) clearly confirms this for 4722. On 

 all three, 94, 95 and 96, there were also taken shutter comparisons 

 showing the relative positions of the components given with one, 

 two and three turns. These all show that the component coming 

 up with decrease of inductance is the one toward the red: the com- 

 ponent toward the violet retains its position while its intensity be- 

 comes relatively less. The effect of removal of inductance is similar 

 to that obtained by moving up to the end of a somewhat longer gap 

 leaving the inductance the same. (See 111b). 



The conclusions to be drawn from the photographic study are: — 



1. That it is impossible by means of the echelon grating to com- 

 pare the positions of maximum density of any but quite monochro- 

 matic sources, whether the condition be either double or single order. 



2. That it is impossible in general to distinguish the images given 



