necessary for Sensibly Perfect Definition. 357 



focusing a small telescope is to move the eye across the eye- 

 piece, altering the adjustment until there seems to be no re- 

 lative motion of object and cross wires. I have tried this 

 plan in an improved form in order to see whether a higher 

 degree of accuracy of adjustment was really attainable, al- 

 though theory seemed to show that no great advance was 

 to be looked for. A heavy pendulum, executing complete 

 vibrations in about two seconds, was fitted up in front of the 

 telescope, and carried with it a screen perforated by a slit. 

 The width of the slit was about a quarter of the entire aper- 

 ture, and the oscillations were at first of such amplitude as 

 just to bring the extreme edges of the lens into play. In the 

 earlier experiments the slit of the collimator was backed by 

 the clouds, a piece of green glass being interposed. This was 

 before I had discovered the remarkably unachromatic character 

 of the instruments, and I was puzzled to interpret the ap- 

 pearances presented. On one side of the focus the relative 

 motion of the image was (as it should be) in the same direc- 

 tion as that of the pendulum, and on the other side in the 

 opposite direction ; but the transition was not well defined, 

 and the image executed evolutions very visible to the ob- 

 server, who at the same time was not able to describe them 

 as swinging in one direction or the other. The effect upon 

 the eye was remarkably unpleasant and fatiguing to watch ; 

 it disappeared when recourse was had to sodium light, and 

 doubtless depended upon the variation of quality in the light. 

 It may be noticed that spherical aberration would show itself 

 by a swinging of the image in a period half that of the 

 pendulum. 



With the soda-flame the adjustment to focus by getting rid 

 of the swinging motion was pretty accurate ; but not much 

 advantage was gained in comparison with a setting by simple 

 inspection under full aperture. As before, the extreme 

 difference in a set of ten was about *02 inch. 



The substitution of white for monochromatic light was in- 

 structive. In either extreme position of the oscillating slit 

 the light was seen to be spread into a spectrum of moderate 

 length, the blue and red being interchanged after each half 

 period. Under these circumstances the cross wires can be 

 made to maintain their position in that part of the spectrum 

 only for which the telescope is focused. If, for example, it 

 be the green of the spectrum, we may bring the cross wires 

 to this position when the pendulum is at rest, and then, in 

 spite of the oscillation, the position will be maintained. If, 

 without altering the focus, we move the cross wires to another 

 part of the spectrum, then, when the pendulum oscillates, the 



