April 10, 1865.] 



♦ KNOWLEDGE 



299 



swamp where the waters have their birth. Leniminkiiinen 

 overcame biiu by the power of words, but in the end 

 had to beat a retreat. 



IS THE DL\:\IETER OF THE PUPIL OF 

 THE EYE AX EQIMVAT.EXT OF 

 THE LIGHT'S IXTEXSITY? 



By John Goruam, M.R.C.S. Ekg. 



(From a paper read before the Royal Society in November, 1884.) 



(^Continued from page 2&5.) j 



TT"* E have next to furnish ourselves with specimens of 

 V T light of ditl'ereut intensities, in order to ascertain 

 whether there is any coincidence between them and the 

 magnitudes of the pupil. We are not long in discovering 

 that the light of the atmosphere, as reflected from the sky, 

 is well adapted for the purpose ; for, although very vari- 

 able in brightness, the variations are characterised by 

 distinct j^eriodicities in accordance with meteorological 

 changes in the atmosphere : hence the variations are defi- 

 nite, and may be anticipated from day to day. These 

 changes occur almost hourly, but they are much more fre- 

 quent and rapid when the evening twilight is succeeding 

 upon the sunset. The subjoined diagram (Fig. 1) shows 

 the effect of the evening twilight upon the pupil when 

 examined every ten minutes con.secutively for two hours 

 from 5 p.m. to 7 p.m. The light chosen was that from 

 a small area of the S.W. sky, about 11° above the horizon. 

 What this experiment is specially intended to show is the 

 remarkable coincidence in the lapid departure of the light 

 during the second hour, and the similarly rapid dilatation 

 of the pupil. Oar ordinary sense of vision pronounces 

 upon the fact of the light's decrease that it is getting sensibly 

 duller every few minutes. Our pupil indicates the same 

 fact, but more accurately, by defining the amount of dul- 

 ness in fractions of an inch. 



Sept. 19, 1884.- 



-Faie, Su:,-nt Day. DuiiETEn 



IN" HP.ND2EOTES OF AN I.NCU. 



o? Tns PcriL, 



S 6 7 8 9 10 11 13 13 li 15 10 17 13 19 20 21 23 S3 24 2,5 2-3 S7 M 



Diameter o' pupil every consecntive ten minutes from 5 p.m. till 7. 

 Fig. 1. 



We may now try the effect of sky light on the magnitude 

 of the pupil every consecutive hour throughout one day. 



These experiments were commenced at four in the 

 morning, and terminated at eight in the evening, after 

 which a standard Sugg's candle was substituted for the 

 twilight, and finally the light of the moon. The whole of 

 these pupil measurements are seen to coincide in a remark- 

 able manner with the changes in the light's brilliancy 

 throughout the day, as well as with the feeble light of the 

 candle, and the still feebler light of the moon. 



The feeblest light registered in the diagram (Fig. 2) was 

 moonlight, indicated by a pupil diameter of •It* in. ; 

 U' xt that of the morning twilight at 1 o'clock, and of the 

 candle, the pupil measuring the same in both. 



Aug. 2, 1884. — Fair, Suxny Day. Dumetees of Pupil in 



HCNPREDTHS OF AN IXCU, VRD'A Ai A.il. IILL 8 I'.M. , 



Diameters of the popil from 4 o'clock in the moraing to 8 in the 

 eveninp. 



Fig. 2. 



In obtaining these results we have trusted to the re- 

 sources of the pupil alone registering its indications, regard- 

 less of any foregone conclusions as to the quality of the 

 light derived from our mere judgment or sense of sight. 

 The question again presents itself to the pupil's indications 

 and its varying intensities of daylight coincide. In no 

 single instance has a discrepancy yet arisen : a dull light 



Oct. 6, 1884. — Diametess of Pupil in Hundeedths of an iNcn. 

 ' Eclipse of Moon. 



o— O = the moon. 



> = daylight. 



Fig. 3. 



having been invariably accompanied by a dilated pupil, a 

 bright light by a contracted one, while with a light of 

 known absolute intensity, as that of a Sugg's candle, the 



