130 SUMMARY OF CUERENT RESEARCHES RELATING TO 



as a test for high-power objectives, and which, not being a diatom or 

 very diaphanous, needs rather the quality of ' resolution ' than that of 

 * definition ' to deal with it satisfactorily. I take it that a ' test ' to 

 be of use should be fairly easily obtainable ; that the specimens 

 should, from the nature of the structure, be uniform ; and that to 

 merit the name of a ' test ' it should not be too easily made out, even 

 by the best modern glasses. 



" I am sanguine enough to think that the statoblast of Lophopus 

 crystallinus, which is easily procurable in any numbers, will be found 

 to meet these conditions. The difficult part is the structure of the 

 membrane, which seems to be stretched over the coarse hexagonal 

 framework of the statoblast. I have seen it well, but it tried my 

 fine ^^5 of Tolles, and was most bright and clear with an excellent 

 -^^ homogeneous-immersion objective, which Mr. Tolles has just sent 

 to me. I found the more axial the illumination the better — obliquity 

 was fatal. I used a cap on my condenser of ■^^, the diameter of 

 condenser being ^, and it evidently aided the definition. 



"While on high-power testing, let me say that the hexagonal 

 areolations seen in the apparent openings in Isfhmia nervosa are 

 valuable for trying the qualities of ^, ^V ) ^^^ tV ^^ more. The areo- 

 lations are not small, but so delicate as not to be seen at all by a poor 

 object-glass, while the better the quality of objective the more clearly 

 can they be made out, till they look like delicate network. I mention 

 this because I find the existence of this delicate structure is not 

 generally known ; though I have used it for some years to try the 

 quality of objectives." 



Microscopical Structure of Malleable Metals.* — The following 

 observations have been made by Mr. J. V. Elsden on the minute 

 structure of metals which have been hammered into thin leaves. 

 Notwithstanding the great opacity of metals, it is quite possible to 

 procure, by chemical means, metallic leaves sufiiciently thin to examine 

 beneath the Microscope, by transmitted light. Silver leaf, for instance, 

 when mounted uj)on a glass slip and immersed for a short time in 

 a solution of potassium cyanide, perchloride of iron, or iron-alum, 

 becomes reduced in thickness to any required extent. The structure 

 of silver leaf may also be conveniently examined by converting it into 

 a transparent salt by the action upon it of chlorine, iodine, or bromine. 

 Similar suitable means may also be found for rendering more or less 

 transparent most of the other metals which can be obtained in leaf. 



An examination of such metallic sections will show two principal 

 types of structure, one being essentially granular, and the other 

 fibrous. 



The granular metals, of which tin may be taken as an example, 

 present the apjjearance of exceedingly minute grains, each one being 

 perfectly isolated from its neighbours by still smaller interspaces. 

 The cohesion of such leaves is very small. 



The fibrous metals, on the other hand, such as silver and gold, 

 have a very marked structure. Silver, especially, has the appearance 



* ' Nature,' xxiii. (1881) p. 391. 



