REMARKABLE EYE-STRUCTURE IN A FISH. 127 



molecular weight is twice the density; hence for helium the 

 molecular weight and also the atomic weight is 2-13 x 2 = 

 4-26. This is again on the assumption that helium is an element 

 and not a mixture. If the atomic weight of argon is 40, then 

 there is no place for it in the periodic system. It has not yet been 

 proved that it is not a mixture. From the spectra it seems that 

 each gas has a common ingredient. The density of helium does not 

 leave much room for the presence of a large quantity of a heavier 

 constituent. To fit the periodic system the density of argon should 

 be diminished by the removal of a heavier constituent, rather than 

 increased by the removal of a lighter one. Again, assuming that 

 the other constituent of argon, if there be one, is a heavier one, 

 and that helium contains the same, then, even if it be present 

 in a very small quantity, it will reduce the density of helium 

 considerably, and consequently bring it much nearer to that of 

 hydrogen. 



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MR. W. Tegetmeier has recently called attention to a fish, 

 which is very curious as regards the organisation of its 

 eyes, of which, like its congeners, it has two, although it 

 well merits the name {tdraophthalmus) that attributes four to it. 

 This fish has extremely bulging eyes, and when it is swimming 

 upon the surface, as is its custom, half of the eye is above the 

 surface of the water and the other half beneath it. Even exter- 

 nally, something abnormal is observed in these eyes. In fact, 

 from the conjunctiva there starts a horizontal band of a dark colour 

 that divides the eye into two parts — an upper and a lower. But 

 the division is more profound still. There is a sort of halving of 

 the pupil so as to form two — an upper and a lower — to which cor- 

 respond a common iris that tends to a division, in the sense that 

 a fold of this membrane separates the upper iris from the lower. 

 But all this would not permit the animal to see in the air as well 

 as in the water, were there not added a special arrangement of the 

 crystalline lens. The crystalline of terrestrial animals has the 

 form of a lens, but, in order to see in water, it requires a nearly 



