to measure their relative intensities and compare 

 them with the known effects of temperature 

 change, etc. Some other methods of interpreta- 

 tion have already been described in our account of 

 solar phenomena. 



The collection of instruments used in the Pasa- 

 dena office-building for the study of the photo- 



Fir.. 54. Pasadena Office-Building. 



graphs includes measuring-machines of various 

 types; visual photometers, for determining the 

 density of the image (and hence the brightness of 

 the object); Koch's registering microphotometer; 

 a Zeiss stereocomparator, for the accurate com- 

 parison of two photographs of the same object; the 

 heliomicrometer, a combined measuring and cal- 

 culating machine for determining the latitude and 

 longitude of objects on the sun; several calculating 



