RECENT ADVANCES IN SCIENCE 195 



energy. This suggests that the sequences G, K, M, and G, 

 R, N may be parallel lines of evolution. 



The results indicate the desirability of further investiga- 

 tion with longer exposures and large dispersion. 



The Threshold of Vision. — A note on recent determinations 

 of the minimum light perceptible to the human eye was given 

 in Science Progress, 12, 552, 191 8. The direct determina- 

 tion by Reeves was based upon observations of an artificial 

 star 1 mm. in diameter from a distance of 3 metres. In 

 Astroph. Journ. 47, 141, 191 8, he gives the results of further 

 investigations as to the effect of the size of the stimulus and 

 the time of exposure on the retinal threshold. The absolute 

 threshold was determined for stimuli varying in size from 

 a 2 mm. square to a 12 cm. square and viewed at various 

 distances. In each case, before taking an observation, the 

 eye was adapted to darkness. It was found that the threshold 

 decreased considerably with increase in the size of the stimulus, 

 but that the total energy entering the eye showed an increase, 

 e.g., for a 2 mm. square at 35 cm., the energy entering the 

 eye at the threshold of vision was 25*3 x io" 10 ergs per sec, 

 whilst for a 12 cm. square at the same distance it amounted 

 to 564 x io -10 ergs per sec. 



Attempts were made to determine the length of time re- 

 quired for a point source to produce a perceptible sensation. 

 For just perceptible intensity, the time was found to average 

 about 2 sees. Independent experiments were made with 

 calibrated photographic shutters placed before the test spot 

 and the brightness necessary for the test spot to be perceived 

 when exposed was determined. In this way, a rapid increase 

 in sensibility up to 2 seconds was obtained. 



The following are a few of the more important papers re- 

 cently published : 



Historical. — Repsold, J. A., Zur Geschichte der Astrono- 

 mische Messwerkzuge. I. Heron's Dioptra, Ast. Nach. No. 

 4931. II. Alte Arabische Instrumente, Ast. Nach. No. 4935, 

 1918. 



Dreyer, J. L. E., On the Origin of Ptolemy's Catalogue 

 of Stars, M.N., R.A.S., 78, 343, 191 8. 



Fotheringham, J. K., The Secular Acceleration of the 

 Sun as determined from Hipparchus' equinox observations, 



