The Romance of the Heavens 49 



But there is a device whereby the power of these giant in- 

 struments, great as it is, can be still further heightened. That 

 device is the simple one of allowing the photographic plate to take 

 the place of the human eye. Nowadays an astronomer seldom 

 spends the night with his eye glued to the great telescope. He 

 puts a photographic plate there. The photographic plate has 

 this advantage over the eye, that it builds up impressions. How- 

 ever long we stare at an object too faint to be seen, we shall never 

 see it. With the photographic plate, however, faint impressions 

 go on accumulating. As hour after hour passes, the star which 

 was too faint to make a perceptible impression on the plate goes 

 on affecting it until finally it makes an impression which can be 

 made visible. In this way the photographic plate reveals to us 

 phenomena in the heavens which cannot be seen even through 

 the most powerful telescopes. 



Telescopes of the kind we have been discussing, telescopes 

 for exploring the heavens, are mounted equatorially; that is to 

 say, they are mounted on an inclined pillar parallel to the axis 

 of the earth so that, by rotating round this pillar, the telescope 

 is enabled to follow the apparent motion of a star due to the rota- 

 tion of the earth. This motion is effected by clock-work, so that, 

 once adjusted on a star, and the clock-work started, the telescope 

 remains adjusted on that star for any length of time that is 

 desired. But a great official observatory, such as Greenwich 

 Observatory or the Observatory at Paris, also has transit instru- 

 ments, or telescopes smaller than the equatorials and without the 

 same facility of movement, but which, by a number of exquisite re- 

 finements, are more adapted to accurate measurements. It is these 

 instruments which are chiefly used in the compilation of the Nau- 

 tical Almanac. They do not follow the apparent motions of the 

 stars. Stars are allowed to drift across the field of vision, and as 

 ^ach star crosses a small group of parallel wires in the eye-piece 

 its precise time of passage is recorded. Owing to their relative fix- 

 ity of. position these instruments can be constructed to record the 



VOL. I 4 



