1884.] Mr. J. Gorham. The Pupil-Photometer. 425 



Vertebrates, except Amphioxus, have a portion of the kidney modified 

 for some unknown purpose not connected with excretion ; that in 

 Cyclostomes the pronephros alone is so modified, in Teleostei the pro- 

 and part of the meso-nephros ; while in Elasmobranchs and the 

 higher Vertebrates, the mesonephros alone gives rise to this organ, 

 which has also, in these forms, acquired a secondary connexion with 

 certain of the sympathetic ganglia. 



IV. "The Pupil-Photometer." By J. GORHAM, M.R.C.S. Eng. 

 Communicated by Dr. WILKS, F.R.S. Received Novem- 

 ber 18, 1884. 



(Abstract.) 



It is agreed on all hands that the pupil of the eye owes its size to the 

 quality of the light ; contracting and dilating according to the inten- 

 sities of the light. It is not agreed, however, if indeed it has ever 

 become the subject of debate, that its magnitudes may be reciprocally the 

 measures of those intensities. There are cogent reasons for believing that 

 they are so, and hence that an instrument which measures the pupil's 

 size, measures at the same time the light's intensity. The photometer 

 was originally constructed for the former purpose only, and indicates 

 the diameter of the pupil in hundredths of an inch. The diameter is 

 found by directing the instrument applied to one eye, with both eyes 

 open, towards a sheet of white paper, or the sky ; the lid of the 

 instrument is now revolved slowly until the two white disks just touch 

 one another at their edges. The decimal fraction opposite the two 

 apertures seen on the scale outside, indicates the diameter of the pupil 

 in hundredths of an inch. 



On examining hourly for several consecutive weeks the light of the 

 day reflected from a given small area of the sky, certain recurring 

 periodicities were observed in the pupil's magnitudes, and these are 

 found to coincide with analogous alterations in the light's intensity ; 

 hence it was inferred that if the pupil owes its size to the intensity 

 of the light, it became from that very fact a measure of that intensity. 

 It is the object of the paper to substantiate this by experiment. To 

 use the instrument for testing light of different intensities we- first 

 set the pupil to a light of a given intensity by using a Sugg's standard 

 candle. This is placed at a distance of one foot from the eye, with a 

 white surface close behind it, in a darkened room. The diameter of 

 the pupil is now taken under the stimulus of the candle flame, and its 

 measure is read off on the scale of the instrument. My own pupil, 

 when impressed with such a light, measures invariably the 0'15 inch. 

 We now place four such candles at two feet from the eye, when the 



