July 1, 1898.] 



SCIENCE. 



13 



A short truncated cone is made of wood, 

 and the conical surface carefully whitened. 

 The approximate dimensions of the cone in 

 my apparatus are : lower base, 20 cm.; 

 upper base, 15 cm.; height, 3.7 cm. The 

 cone is cut in two along the axis, one-half 

 reversed in direction, and the halves fas- 

 tened together in this new position. The 

 whole is then mounted so as to rotate about 

 the axis of the cone, which is placed parallel 

 to the photometer bar. 



The figure represents the photometer. 

 L and L' are the lights to be compared. 



careful measurements of the distribution of 

 brightness in the spectrum, seems to have 

 used a method of observation closely akin 

 to the flicker method, though apparently 

 without perceiving its definite character or 

 possibilities. His well-known color-patch 

 apparatus places side by side two differ- 

 ently colored fields, the brightness of one of 

 which can be rapidly varied by opening or 

 closing the apertures in revolving sectors, 

 I quote the following from his Tyndall lec- 

 tures, delivered in 1894 : 



" By gradually diminishing the range of 



>4 



/-'. 



The cone, cut and reversed as described, 

 revolves about the axis AB. The plane of 

 division of the cone, as the figure is drawn, 

 passes through this axis and is perpen- 

 dicular to the paper. O is the eye-tube. 

 When the instrument is in the position 

 shown, I is in the field of view and is il- 

 luminated by the lamp L, but when the 

 cone has made one-half a revolution V is 

 seen, which is lighted by the lamp L'. 



While this instrument is compact and 

 convenient for most photometric measure- 

 ments, the disk form is superior for the 

 comparison of pigments, as one of the fields 

 consists simply of a card, which can be re- 

 moved in an instant and replaced by 

 another. Thus differently tinted papers 

 can be easily compared in succession with 

 the revolving disk, which may be of any 

 color, but which in practice is generally 

 white or gray. 



I do not know whether it has been gener- 

 ally remarked that Captain Abney, in his 



the ' too open ' to ' too close ' apertures we 

 arrive at the aperture where the two colors 

 appear equally bright. The two patches tuill 

 cease to ivink at the operator, if we may use 

 such an unscientific expression, when 

 equality in brightness is established. This 

 operation of equalizing luminosities must 

 be carried out quickly and without concen-. 

 trated thought, etc." 



It seems probable that Abney, throughout 

 these measurements, applied himself not so 

 much to a careful comparison of the bright- 

 ness of the two colors involved as to re- 

 ducing to its faintest condition this luinh, 

 which differs little in its nature from a 

 flicker. 



Frank P. Whitman. 



BIOLOGICAL SURVEY OF LAKE ERIE. 

 On July 1, 1898, the U. S. Commission 

 of Fish and Fisheries will inaugurate a bi- 

 ological survey of Lake Erie, under the di- 

 rection of Professor Jacob Reighard, of the 



