Adjusting the Image of a Laboratory Light Source. 317 



follows : the slit S is illuminated by means of an ordinary 

 incandescent lamp placed within 1 or 2 yards from the 

 former. Its image T is then directly viewed by the unaided 

 eye, or if greater accuracy be desired, by means of the small 



Fig. 1. 



i 





Illustrating' method of focussing image of Light Source 

 on slit of Spectrograph. 



magnifying lens L. The electrodes E and E' are then 

 moved into position and adjusted until the slit image and 

 spark-gap are seen to coincide. Any region of the spark- 

 gap can in this way be brought into coincidence with any 

 part of the slit image without much trouble. Accuracy of 

 focus is tested by displacing the eye sideways, and the least 

 deviation from coincidence will immediately reveal itself by 

 a parallax effect between the slit image and the tips of the 

 electrodes. If the source be a flame the orifice of the burner 

 may be brought into coincidence with the slit image, or, if 

 the region to be examined be too far above the burner a 

 piece of wire held in that region will serve the purpose. 



Evidently the same method applies when it is desired to 

 adjust the position of the objective 0. In this case the 

 spark-gap between E and E' is placed in line with the optic 

 axis of the collimator at a distance from the slit which 

 corresponds to the focal length of and the size of image 

 required. The slit image T is then made to coincide with 

 the spark-gap by adjusting the position of the objective 0. 



Manchester, May 1920. 



