[ 425 ] 



LVIII. Remarks suggesting doubts respecting the received value of 

 fi, the Index of Refraction of the Lens. By Robert Tem- 

 pleton, Esq.* 



[With a Plate.] 



1. A SERIES of very small holes being perforated in a thin 

 -^- sheet of brass in a gradation of sizes varying from about 

 one-tenth of a millimetre to about fifteen tenths of a millimetre 

 in diameter, if minute dots on a slip of cardboard be examined 

 through them, a set of measurements can be taken of the dis- 

 tances at which the most perfect vision of the dots is obtained. 

 A few trials will be required with an unpractised eye, as the ob- 

 server does not at first feel quite sure at what distance the object 

 is best seen ; but very soon a set will be obtained such as is con- 

 tained in the third column of the annexed Table : — 



No. 



8. 



D. 



L04. 





millim. 



millims. 





1 



•15 



28 



77289 



2 



•25 



43 



77644 



3 



•32 



63 



77059 



4 



•55 



102 



7-7318 



5 



•81 



150 



7-7324 



6 



1-00 



165 



7-7825 



7 



1-07 



190 



77506 



8 



1-52 



260 



7-7668 



2. The measurements in this column are a single set, taken in 

 a well-lighted room, at a time when the sun was high but not 

 shining into the room. As far as has been observed, they are 

 constant for the same degree of light f. 



3. The diameters of the small holes given in the second 

 column being divided by the distances corresponding to them in 

 the third, the logarithms of the quotients are placed opposite to 

 them in the fourth column. These logarithms are perceived to 

 be as nearly constant as can be expected, it being somewhat dif- 

 ficult to settle upon the precise point of perfect vision within a 

 certain range of tolerably distinct vision; but the smaller the 

 dot, consistent with exact definition, the more readily is the exact 

 distance of perfect vision determined. We find, then, that for 

 each hole there is a distance of quite perfect vision, a nearer or 



* Communicated by the Author. 



t Some of the experiments here repeated were made many years ago, in 

 an attempt to show that the adaptation of the eye to vision at different 

 distances could only doubtfully be attributed to an exercise of muscular 

 power, since a limit ought to be arrived at, at which the effort to maintain 

 vision would cause it to become more or less intermitting. 



