286 



Mr. A. A. Michelson on the Application of 



/&, 



V (calc). 



•0 . . . 



1-00 



•1 . . . 



. -94 



-2 . . . 



•75 



•3 . . . 



•50 



•4 . . . 



•24 



•5 . . . 



•00 



•6 . H . 



•15 



•7 . . . 



. -22 



•8 . . . 



. -19 



•9 . . . 



. -15 



.•0 . . . 



. -oo 



observations, the results agree fairly well with theory. If <£ 

 is the ratio of the wave-length to the width of the rectangular 

 aperture, the above formula becomes 



y_ sin27r<fr/(fro 



27r</>/0 o ' 



from which the second column in the following Table was 

 calculated. 



cj)/cf) . V (calc). V (obs.). 



0-0 ... . l'OO i-oo 



94 

 73 

 40 

 13 

 09 

 10 

 09 

 07 

 05 

 04 



From this table it appears that if the visibility is to be 

 estimated by observations with a telescope of 12 millim. aper- 

 ture (or with a circular aperture about one fourth greater), an 

 error in the adjustment of the surfaces of a second of arc 

 would produce a diminution of 4 or 5 per cent, in the visibility. 

 Accordingly, if the ways on which the mirror-carriage moves 

 are not true to this degree, it is necessary to make the adjust- 

 ment for every observation. 



This can be done with very great accuracy by moving 

 the beam of light from side to side and adjusting the mirror 

 until there is no perceptible alteration in the size of the rings. 

 Since the admissible error in adjustment is inversely propor- 

 tional to the aperture, the observations may be facilitated by 

 making this as small as possible if there be light to spare. 

 This is all the more necessary for the same reasons, if the 

 surfaces be not true. However, the error due to this source 

 may be easily corrected (since all the observations are affected 

 alike) by multiplying by a constant factor. 



In order that the visibility-curve may extend as far as pos- 

 sible, it is necessary that the vapour should be very rare. 

 Accordingly, in all but a few cases to be mentioned later, the 

 substance to be investigated was enclosed in a vacuum-tube, 

 which was previously heated to drive off any moisture or 

 occluded gases. 



The vapour was rendered luminous by the discharge from 



