166 SECTION PHYSICS. 



orbit is inclined to the earth's equator ; and, consequently, if we have 

 placed the moon centrally in the telescope adjusted to follow her in 

 right ascension, after a time she will have moved up or down (in 

 declination) some distance, in consequence of her orbital motion. 

 The motion of the moon in declination, as it is called, is greatest 

 when she is near her nodes, and then in one minute she may move 

 as much as 16 seconds of arc, consequently in 10 seconds she may 

 have moved 2*7 in arc, or nearly 3 seconds of arc ; and as we can 

 depict on the moon objects whose diameter is not greater than 

 i second, which is equal to about a mile, it is very clear that when 

 the moon has the greatest motion in declination, the lunar pictures 

 cannot be as perfect as when she is at a greater distance from her 

 nodes. The Astronomer Royal, Sir George B. Airy, whose name is 

 not only connected with mathematical astronomy, but also with astro- 

 nomy as a great mechanist, has proposed to add to the ordinary 

 equatorial a second axis, which would be carried round so as to remain 

 parallel with the axis of the moon's orbit in its diurnal path, and pro- 

 vided with an independent clockwork driver carrying the telescope 

 in a direction inclined to the equator, so as to follow the moon in her 

 orbit from west to east. I need scarcely say that this appliance 

 would complicate a large telescope so much that it would be almost 

 impossible to adopt it ; still it is conceivable that we may, besides the 

 motion in right ascension, give an adjustable motion to the declina- 

 tion axis, varying from nothing up to 16 seconds of arc either in 

 north or south declination in a minute. But this has not yet been 

 done, and consequently it has to be accomplished in order to make it 

 possible to photograph the moon perfectly at all times. However, 

 the first photograph I ever obtained I took with the piece of apparatus 

 I hold in my hand, by which I followed the motion of the moon both 

 in right ascension and declination. It is unfortunately now getting 

 broken. It was put on to the eye end of the telescope which was 

 allowed to remain at rest, and by means of this milled head, the 

 sliding piece carrying the sensitive plate having been placed so as to 

 move in the direction of the motion of the moon in her orbit and in 

 right ascension, I was able for a short time, by making three quarters 

 of a turn, to follow the moon by hand ; and here is a picture which 

 was obtained in 1853 in that way. But I was not the first to photograph 



