276 



Dr. G. W. Royston-Pigott. 



If the spider-lines be employed, careful microscopic examination 

 should be made and their' actual thickness measured. When they 

 cannot be exposed, a very long focus objective must be employed with 

 a deep eye-piece and a long tube. It should be noted whether they 

 are simple or complex in structure. 



Method of Estimating the Size of a Minute Molecule or Line. — Contact 

 is apparently made ; first, with the outside of the spider-lines, the 

 micrometer is then read. Inside contacts must now be noted. The 

 mean will be the true size of the molecule, whether the spider webs 

 are of the same thickness or are of different thicknesses.* 



If a fine membrane be folded over tightly, the line of flexure 

 appears exceedingly black and straight, and somewhat thicker than 

 the membrane. The folded edge may then be considered at any point 

 as a refracting semi-cylinder, of special optical beauty and precision of 

 form unrivalled in organic life. 



To ascertain roughly the nature of this black line, gold beater's skin 

 was folded over and pressed ; at the line of flexure a very sharp black 

 line appeared, when examined with a pocket lens. This line appeared 

 about one-third thicker than the free edge of the skin forming a section. 

 Gold leaf exhibits similar dark lines of flexure, f upon the malachite 

 green of the gold under transmitted light, if there be but little alloy 

 present. On the whole, this line is thicker than the leaf, or about 

 1 -100,000th, but it is not nearly so sharply displayed as in the flexure 

 of insect scales ; in many cases of which the breadth of this black line 

 is even less J (particularly in the finest gnat and Podura scales). 



Example. — In the gnat's scale the membrane, including the mole- 

 cules of its structure, stretches in graceful little curves from spikelet 



* If R be the first, R/ the second reading d, d', the thickness of the webs, it is 

 evident, if A be true diameter of molecule, 



E + E / = 2A, 



T£ + TV 

 A =-— - — the mean, 

 2 



which is independent of the size of the webs. 



f One grain of gold can be hammered into leaves sufficient to cover 56^ square 

 inches ; its thickness will then not exceed l-282,000th of an inch. — "Encyclopaedia 

 Britannica," Article " Gold." The skin is really a tough French paper. 



X This line has in general resisted every attempt at resolution ; yet, when a cor- 

 rugated scale is folded over, not too sharply, a serrated edge must necessarily exist. 

 I have never been able to define tbis serration perfectly, except with object-glasses of 

 the highest excellence and reduced aperture, which of course darkened and sharpened 

 the defining shadows. A very fine view was obtained by means of the posterior 

 focus of a l-50th being shortened to about two inches, which very considerably 

 reduced the aperture and admitted considerable separation of the front lenses, to 

 correct the aberration thus introduced. 



