﻿296 M. H. Wild on the Absorption of Light by the Air. 



then that the apertures of the pupils of the observer's eves are 



beTeX KvT V b ° th f SkS V Th * fi '' St C ° ndltion *£ Z o y 

 is strttlv ^ *^**™*™*J Precautions; and the second 

 is, strictly speaking, never fulfilled. It is one of the oldest oh 

 servations in reference to the accommodation o h eye that the 

 e P nlied C °onToT OW ? wl f> a ~odated for near ol 'eets, aud 



of tne 8 nunn IT " '? ^fT" Allo ™S fo ' *i. alteration 

 ot the pupil, the above formula becomes more exactly 



order to ascertain the approximate value of the fe£ of ^ec- 

 tl0n (a) *' X est ™ated the alteration in the pupillary aper- 



o72 e nn n f Pa rl nS fr ° m the accomm °d«tion to an object at a distance 



cln.es about Z *•$** \T' ?* I ( ° md that the dia ^ 

 cnanges about 6 milhm. If, therefore, we put o=2-4 mil 



>ms A = 3 mfihms. Introducing these numbed into the above 



hSrf 2 a 00Of Ul f Dg ^ C ° effiC / Ut " fr0m the observation at a 

 height of 2000 feet we get, instead of the above value «=0-9029, 



«=0-7225. 

 d ei T able! nflUenCe ° f the alterati ° U in tbe ^ a is th ™ f ™ onsi- 



the T ew e tt™W IT 6 "' aS We " astl >e uncertainty in ascertaining 

 value of tL^h dlSapPearanCe ° fthe black circ H diminish the 

 riaUv that !t , o P I n an ° meter 3S a measuri «S-i^trument so mate- 

 science ^ C ° meS ° P t0 the P '' eSent re q™ements of 



A De la Rive has recently again taken up the investigation of 



Sat JIT 07 ° fth r r J a,ldfo,lowed the'only rationaTpath- 

 hat of endeavourmg to determine photometrically the ratio of 



obse"er SPa Tr y0f f tW ° ° bJeCtS ^ diffel ' ent dista ° ces *»» *« 

 obseiver. The instrument in question was exhibited at the 



Meeting of the Sw.ss Naturalists in Geneva in August 1865 



felt A 88 " 6 tlme the , ab ° Ve the0r y 0f the c auses of the varying 

 degree of transparency of the air was propounded*. De la Tfvf 

 has not, however, as yet published anf results of observation 



Before I knew of De la Rive's work, having become posse sed 

 of some long wide tubes for other investigations, I m«£TS 

 * Compare Phil. Mag. S. 4. vol. xxxiv. p. 241. 



