502 Intelligence and Miscellaneous Articles. 



previous Notes, we have the following numbers, all referring to the 



air-thermometer : — 



Silver 954° Palladium 1500 



Gold 1035 Platinum 1775 



Copper 1 054 Iridium 1950 



— Comptex Rendu* de V Academic des Sciences, t. lxxxix. No. 17, 



pp. 702, 703. 



RESEARCHES ON DALTONISM. NOTE BY J. MACE AND W. NICATI. 



I. Thanks to the kindness of the Principals of the Lyceums of 

 Marseilles and Grenoble, and of the Principal of the College of Aix 

 in Provence, we have been able to examine a total of 925 boys, of 

 whom 33 were daltonians, or 3*57 per cent. We have examined a 

 smaller number of girls, or 241, of whom one only was daltonian. 

 Por these elementary researches we have emploved Seebeck's me- 

 thod so ingeniously improved by M. Holmgren, consisting in getting 

 the examinees to select, from a mass of specimens o£ coloured wool, 

 those which are like some suitably chosen types, without troubling 

 about the names, more or less accurate, that may be given to the 

 colours. These researches have extended to too small a number of 

 individuals to possess any real statistical value ; nor was that our 

 aim. They are nevertheless to a certain degree interesting from 

 their agreement with the results obtained by Holmgren, Jeffries, 

 and others abroad. On the other hand, they are utterly different 

 from the results obtained by Dr. Pavre, who finds from 20 to 30 

 daltonians per hundred individuals examined. 



II. The principal object of our investigations has been to obtain 

 comparative measures between the quantities of light perceived in 

 the different parts of the spectrum by the daltonian and by the 

 normal eye. In the experiments which we relate today we em- 

 ployed an indirect method. It is based upon the fact that the 

 acuteness of vision diminishes simultaneously with the intensity of 

 the light ; and it consists in measuring the visual acuteness of the 

 daltonian in the different portions of the spectrum, always comparing 

 it M'ith the visual acuteness, in the same circumstances, of a normal 

 sight. We make use of a spectrum with a total length of more 

 than 5 decimetres, thrown upon a stretched graduated rule of black 

 velvet. Over this rule slides at will a square of white cardboard, 

 from which a letter of the alphabet is cut out 5 millims. in diameter. 

 (Letters written with ordinary ink fluoresce very inconveniently in 

 the violet.) The measuring consists in ascertaining the maximum 

 distance d at which the daltonian must place himself in order to 

 distinguish the character, and immediately afterwards the corre- 

 sponding distance D for one of us, always the same individual. 



Determining in this way the values of the ratio — for the various 



regions of the spectrum, we have obtained curves which may be re- 

 ferred to three types : one set, three in number, descend towards 

 the red end of the spectrum ; another curve descends towards the 



