46 



the daytime to determine the position of the instrument, 

 and another objection is found in the greater uncertainty 

 attending the transit of the sun's limbs, which I think we 

 can see on the screen. 



We have here a beautiful photograph taken from the 

 sun directly, and for which we are indebted to the skill 

 of Lewis M. Rutherford, Esq. You will notice that the 

 rounded limb of the sun cannot be so nicely bisected as 

 can the image of this star which follows afterwards. 



Let us now examine the method of noting the transit of 

 a star across a wire. If I take this chronometer or that 

 clock, I can count the beats as I sit with my eye to the 

 telescope ; and as the star crosses each wire I can note the 

 second and the fraction of a second, and a skilful observed 

 will. only on rare occasions estimate this fraction a fifth of 

 a second in error. It is better, however, to lessen the 

 errors which depend upon the personality of the observer, 

 such as his observing too fast or two slow, and to econo- 

 mize the time of writing down the observations, to record 

 them automatically by means of the chronograph, an in- 

 strument first used in this connection by an American 

 astronomer. We have a small one before us, and you see 

 it consists of a metallic cylinder around which a sheet of 

 paper is coiled, which is revolved uniformly by clock 

 work. A glass fountain pen rests upon the surface of the 

 paper, and as the cylinder revolves the pen draws a line 

 upon it. Now if you conceive that this cylinder be slowly 

 moved along at the same time that it revolves, you will 

 understand that the pen never marks over the same part 

 of the paper. Now suppose that this cylinder rotates just 

 once in sixty seconds, and suppose that I cause this clock 

 by means of an electric circuit to slightly move the pen at 

 the beoinninu: of each second ; this will cause a slight 

 notch in the line, which registers upon the paper the 



