1896.] 



for a form of spectroheliograph. 



181 



Some modification of Littrow's spectroscope with fixed slits 

 would seem obviously the simplest one to use in these investiga- 

 tions. I am under the impression that Mr Hale has recorded his 

 rejection of this form, but I cannot find such a reference. Recent 

 work with a modification of Littrow's spectroscope leads me to 

 believe that the difficulty arising from reflexions from the lens- 

 surfaces can be overcome by the use of suitable stops. The spectro- 

 genic apparatus here suggested is accordingly a modification of 

 Littrow's spectroscope. 



Modification of Littrotu's Spectroscope. 



The essence of this form is that one tube and lens serve both 

 for collimator and for camera or observing telescope. 



1 1 



Slo7l> Motion for Grating 







rjrr^m 





ir-tf 



Focussing Screz 



it 



The tube T serves as collimator and camera ; an object-glass L 

 is fitted in a tube T' which slides in the main tube T for focussing 

 purposes. 



A slit A is fitted at one side of the tube T, its length being 

 perpendicular to the axis of the collimator ; a reflector B placed 

 near the axis reflects light normally incident on the slit so that 

 it passes along the axis of the collimator and so through the 

 object-glass. 



A diffraction grating (or a train of prisms with a reflector 

 at the end) is attached properly to the tube T by means of a 

 framework K. Light, after falling on the grating (or after a 

 double passage through the prisms), falls on the object-glass 

 again and passes along the tube T till it falls on a second re- 

 flector E, set so as to reflect the light out through the slit F, 

 which is fixed opposite the first slit. The arrangement is in a 

 certain sense direct vision; and the line joining the middle points 

 of the two slits may be called the line of direct vision; it is of 

 course perpendicular to the axis of the collimator. (Such a form 

 of instrument would be convenient for ordinary spectroscopic 



