Popular Science Moyithly 



increase of current, due to light, by means of an 

 ammeter or galvanometer. However, there are 

 several difficulties in this simple process. When 

 selenium is exposed to a strong light, some 

 minutes or even hours are required for the re- 

 sistance to return to its original value. Selenium 

 is extra sensitive to red light, and so does not 

 give directly a measure of how bright a light 

 would appear to the eye. For instance, a carbon 

 filament lamp with its yellowish light will affect 

 a selenium cell just as much as a much whiter 

 tungsten lamp of double the candle power. 

 Finally and worst of all, selenium is very ir- 

 regular in its action, and no experimenter has 

 yet solved to his own satisfaction the 

 mysteries of this element. 



The Selenium Cell Is Packed in Ice 



As applied to the stars, however, many 

 of the ordinary difficulties disappear. 

 Star light is so faint, even at the focus of 

 a large telescope, that the slow recovery 

 of the selenium is not such a 

 drawback; next, since we are 

 usually concerned only 

 with variations of light, 

 it matters little which 

 color is used ; and 

 lastly the irregu- 

 lar action may be 

 controlled some- 

 what by keeping 

 the selenium at a 

 low uniform tem- 

 perature. Strange 

 as it may seem, 

 when the light of 

 stars is to be meas- 

 ured, a selenium 

 cell at the end of 

 the telescope is 

 surrounded with 

 an ice pack, the 

 ice being renewed 

 every day in sum- 

 In the circle is shown 

 a selenium cell and ice 

 pack attached to the 



telescope 

 The oval diagram 

 above the circle pic- 

 tures the system of 

 Delta Orionis, show- 

 ing orbit, eclipse 

 positions, and compa- 

 rative size of the sun 



To the right — Un- 

 mounted selenium 

 cell, natural size 



