130 



SCIENTIFIC RECREATIONS. 



glasses, quite free from blemishes, had shown me that these hexagons had 

 round points, contrary to the descriptions of micrographs. These observa- 

 tions, corroborated by the micrographic photographs of Lackerbauer, the much- 

 regretted designer, and by Colonels Woodward and Washington, left not the 

 slightest doubt that it was necessary to discover why the eye persistently 

 saw hexagons where there were circles. To elucidate this point, it was 

 necessary to find some means of reproducing artificially what nature had 

 accomplished with so much precision on the surfaces of algae. After many 



fruitless attempts, I decided on making a 

 trial of a stereotype plate covered with dots 

 arranged in quincunxes, very close together" 

 (figs. 132 and 133). "The result was more 

 successful than I had hoped ; the effect pro- 

 duced is exactly that of the arrangement of 

 the so-called hexagons of the most beautiful 

 of the algae, the Pleurosigma angulata. If 



Fig. 133. Another figure of the same kind. , f. 



these stereotypes are examined with one eye 



only, we shall be immediately convinced that we have to do with hexagonal 

 polygons." It is useless to give any long exposition of a figure so clearly 

 explanatory ; it is simply an effect of the contrast and opposition of the 

 black and white in the sensation of the retina. This effect is particularly 

 striking with fig. 134, a negative photograph heliographically engraved 

 according to fig. 133. In this the white points seem to destroy the black 

 spaces, and to approach each other tangentially, and the irradiation is so 

 intense that the white circles appear much larger than the black of fig. 133, 

 although of the same diameter. There are in these facts many points 

 which may interest not only students of micrography, but also artists. As 

 to the algae, the origin of this investigation, it remains to be discovered if 

 these circles which cover their silicious carapace are the projection of small 



hemispheres, or the section of openings 

 made in the thick covering. Certain 

 experiments, however, seem to prove 

 that they are hemispheres, and the 

 theory is also confirmed by a micro- 

 scopic photograph from Lackerbauer's 

 collection, magnified 3,000 diameters, 

 in which a black central point is seen 

 in the centre of each circle, a certain 

 reflection of the luminous source re- 

 produced in the focus of each of the 

 small demi-spheres which constitute 

 the ornament of the algae. The microscope, which has progressively shown 

 first the streaks, then the hexagons, and then the round points, will surely 

 clear up the point some day or other. 



Mr. Silvanus P. Thompson, Professor ot Physics at University College, 



Fig. 134. Third figure. 



