12 Dr. H. Schlagintweit's Observations in the Alps 



image, similar to that formed by the intimate mixture of a white 

 and a black powder. The distance at which this occurs is, as 

 might be expected, different for different eyes j the possible error 

 in judging of the transparency of the atmosphere is, however, 

 greatly lessened by the circumstance, that only the ratio of the 

 distances from both discs is to be taken into account, and the 

 short-sighted eye will observe the disappearance of the small 

 black circle also at a much smaller distance. While considering 

 the angle under which the circles disappear, it may not be unin- 

 teresting to introduce the results obtained by the labours of 

 Hueck* upon this subject. They may be stated as follows : — 



1. A normal eye, which can accommodate itself to all distances, 

 observes the disappearance of small objects, whether they be near 

 or distant, under the same angle of vision. 



2. With larger objects the angle necessary to their recognition 

 increases a little. [The absence of perfect transparency seems 

 to have made itself appreciable here.] 



3. A stroke is seen further than a spot whose diameter is 

 equal to the breadth of the stroke. 



4. White objects on a black ground are better seen than when 

 the arrangement is reversed. 



5. The smallest angle of vision under which black spots on a 

 white ground were visible amounted to 2' 6" ; whereas for white 

 strokes on dark ground it was 1' 2". A spider's thread was seen 

 by Hueck under an angle of 0' 6" ; a shining white wire under 

 an angle of 0' 2". In our case No. 2 and No. 3 are especially 

 worthy of attention. White objects on a black ground appear 

 more striking, because the impression made by a luminous sur- 

 face upon the retina spreads itself laterally by irradiation f. [For 

 this reason, slopes which are partly covered with snow appear at 

 a distance much more uniformly covered than they really are.] 

 Hence the necessity, if we would obtain comparable results, to 

 use the same illumination in all our experiments. It was a.matter 

 of indifference whether we used direct sunlight or chose the 

 shade. We preferred the latter, as we knew we should always 

 have it in our power to shade the instrument. This enabled us 

 also to make our own choice as to the direction in which we 

 receded from the diaphanometer. Not unimportant for the 

 obtaining of comparable results is the precaution, that the eye 

 should nut be wearied by continual fixation upon the disc; it is 

 better to rest the eye from time to time upon suitably dark- 

 coloured objects ; and thus be certain that the black circle does 

 not disappear too soon. We will remark, lastly, that the direc- 



* The Motion of the Crystalline Lens, by Dr. A. Hueck. Leipzig, 1844. 

 t This subject is treated in a profound and elaborate manner by Plateau 

 in the Me'moires dc V Aeadt'mie dc Bruxclles, vol. xvi. 



