the Pupil of the Eye in the Dark. 91)7 



diameter the greater the magnifying power, as depending 

 upon the eyepiece. 



If the object in view is a very faint star or nebula in the 

 case o£ a telescope, or an ultra-microscopic subject in the 

 case of a microscope, the field is necessarily very dark. All 

 side light must be carefully avoided, and the pupil of the 

 eye is, without doubt, in a highly expanded condition, so 

 that the eyepiece employed may have an orifice smaller than 

 the pupil. The object of this communication is to show how 

 the diameter of the pupil may, with fair accuracy, be measured 

 in such all but complete obscurity. 



The accompanying sketch will illustrate the means employed 

 to effect this. 



Fiar. 1. 



The apparatus consists of what is virtually one tube of a 

 common opera-glass, with the eyepiece removed. Into the 

 sliding tube is inserted another tube B of suitable length, 

 and this is pushed home until it is stopped by the offset at 

 the far end of the sliding tube. With the apparatus at its 

 longest the distance from the open end to the near side of 

 the object-glass may be measured, and also the exposed 

 portion of the sliding tube. In any other position the defect 

 in the latter is equal to that in the former, the value of which 

 is thus at any time readily determined. 



In the cell of the object-glass and in contact with it on its 

 near side, is inserted a screen made of the black opaque 

 paper used by photographers. Two small pinholes PP' are 

 made upon one diameter, at equal distances from the axis 

 AF of the lens. F is the principal focus of the lens. The 

 focal distance AF requires to be known, and it is a quantity 

 which can be determined with considerable accuracy. 



The instrument is directed upon a distant and solitary 

 light when the night is dark. A pair of fine beams will 

 then pass through the pinholes PP', and converge to the 

 focus F. 



These will both be visible to an eye placed coaxially with 

 the lens, if the apparent diameter of the pupil of the eye is 

 not less than the distance apart of the beams in the position 

 along the axis which the apparent pupil happens to occupy. 



