Intelligence and Miscellaneous Articles. 503 



violet end ; and two present a minirauru of perception in the green. 

 (We here give only the observations made upon eyes of absolutely 

 normal refraction and visual acuteness, and consequently giving the 

 surest guarantees of correctness.) We must particularly insist upon 

 the instances of descent of the curve in the green, which we believe 

 we are among the first to verify with certainty. Here are the 

 figures of one of the experiments for a length of 7 centims. of the 



spectrum taken in the interesting region: — — =1*09, 0*83, 0*76, 



1*02. The minimum of acuteness sensibly corresponds to the 

 middle of the distance between the lines E and E in the green. In 

 this latter point d and D differ by 6 decimetres ; and fourteen 

 measurements effected over the whole extent of the spectrum give 

 a perfectly regular curve. 



Let it not surprise that in the observation just reported the visual 

 acuteness in the defective region still corresponds to about three 

 fourths of the normal. Direct experiments have shown us that the 

 visual acuteness varies slowly when the light diminishes in a consi- 

 derable proportion. Thus it was that, in one of the experiments 

 made on this question, the intensity of the light varying in the ratio 

 of 1 to -i, the distance at which one of us had to place himself to 

 recognize the character varied only in the ratio of 1 to 1-34. We 

 have not, however, been able to discover any simple relation between 

 visual acuteness and intensity of light; and we are engaged in 

 transforming the present apparatus so as to leave the visual acute- 

 ness constant while making the intensity of the light alone to vary. 



III. It follows from these first observations that the observed 

 facts are not at all in contradiction with the theory of Young and 

 Helmholtz on the perception of colours. In fact they correspond 

 to the three types of daltonians (for the red, for the violet, and for 

 the green) foreseen by that theory. 



We have controlled the obtained results by comparing the visual 

 acuteness of the normal with that of the daltonian eye when both 

 look through one and the same coloured substance. A red glass, 

 for example, which scarcely lessens the sight of a normal eye, dimi- 

 nishes extraordinarily that of the daltonian for the red ; and a 

 green glass and a blue or violet glass do the same for the other dal- 

 tonians. This procedure, very delicate and not very expensive, can, 

 up to a certain point, be substituted for the method that consists in 

 measuring the visual acuteness by means of variously coloured 

 alphabets. — Comptes Rendus de VAcademie des Sciences, Oct. 27, 

 1879, t. lxxxix. pp. 716-718. 



ON A NEW HYGROMETER. BY FR. SCHWACKHOFER*. 



Eirst a determined volume of the air to be examined is enclosed 

 in a burette, and thence conducted into a second vessel, in which 

 the aqueous vapour is absorbed by concentrated sulphuric acid. 

 The dry air is returned to the burette, and the change of volume 

 measured. The volume-hygrometer gives very accurate results 

 * Zeitschrift d. allg. osterr. Apotheker- Vereiiis, 1879, No. 2, p. 2. 



