RECENT ADVANCES IN SCIENCE 553 



star is of the sixth magnitude. This is certainly over-estimat- 

 ing the limit of brightness of the faintest star visible on the 

 background of the sky, and on a perfectly dark background 

 still fainter objects will be visible. Russell estimated that on a 

 perfectly dark background the limiting magnitude would be 

 8 m .5. He further used Steavenson's determination of the 

 diameter of the pupil. The determination by Reeves of the 

 minimum visibile is more direct and therefore probably also 

 more accurate ; he used an artificial star behind a neutral 

 dyed gelatine wedge, the position of which could be varied, 

 and the proportion of light transmitted calculated. His 

 value corresponds to a star of magnitude 7 m .4. He redeter- 

 mined the diameter of the pupil by Steavenson's method, and 

 obtained a slightly smaller value, which was liable to variation 

 for different observers and according to the state of fatigue of 

 the eye and the general bodily health of the observer. The 

 final results of the three determinations are : 



Area of pupil. Minimum perceptible 



^ ^ energy entering eye. 



Sq. cm. Ergs per sec. 



Ives .... 0*28 38*0 x icr 10 



Russell . ... o"57 77 x icr 10 



Reeves . . . o"5i 18*9 x icr lu 



It is not stated by Reeves whether direct or averted vision 

 was used in his determination. For barely perceptible objects, 

 vision is mainly by the rods of the retina and the retina is 

 most sensitive about midway between its edge and the fovea 

 centralis ; an object which is visible by averted vision may 

 therefore not be visible by direct vision. The smaller value 

 obtained by Russell seems to point to his estimate of 8 m .5 

 being too faint. 



A further note on the same subject by Henri Buisson has 

 recently appeared in the Astroph. Journ. 46, 296, 191 7. He 

 determined the brightness of the faintest visible object by 

 employing a screen rendered faintly luminous by radium 

 paint, the colour of which corresponds nearly to the maximum 

 of sensibility of the eye in the spectrum. The screen was 

 standardised by comparison with a film diffusing by transmis- 

 sion. The results give for the limit of visibility a star of 

 magnitude 8 m . Using the values for the area of the pupil 

 adopted by Russell and Reeves respectively, the values 

 obtained on this basis for the minimum perceptible energy 



