Structure of Actual Crystals. 99 



As regards grease it may be worth while to mention the 

 result of a few observations on greasing one side of a "lantern ' r 

 glass plate of 68 sq. cm. area (3| in. x 3^ in.). Two such were 

 opposed in the balance, one face of each being greased alter- 

 nately over its whole area with heavy marks from a finger 

 which had touched the hair. The difference between the 

 weights of a plate with one face thus greased and clean 

 came out about ^ mg.* The greased area here would exceed 

 that of both faces of one of the mica plates. 



Not feeling quite satisfied with the edges formed by the 

 knife, which under the microscope compared unfavourably 

 with the original edges. I determined on another set of 

 measurements in which the new edges were made by scissors. 

 The objection in this case is that it is less easy to cut straight, 

 but I thought that the prejudicial effect might be obviated 

 by cutting the two pieces together superposed. The mea- 

 surements were conducted as before except that now four 

 measurements of the overlap were taken along each edge. 

 But the discrepancy resulting from a comparison of the 

 relative weights and areas was not removed, being indeed a 

 little greater than had been found before from what were 

 practically the same pieces. 



The total thickness above reckoned is about 50 wave- 

 lengths (X) of soda light, so that a difference of a thousandth 

 part corresponds to AX. But as regards the formation of 

 interference-bands in the optical examination, this difference 

 must be multiplied by 3, being doubled in virtue of the 

 double passage of the light reflected by the hinder surface, and 

 increased again in the ratio 3 : 2, or thereabouts, in virtue of 

 the refractive index of the material. The discrepancy sug- 

 gested bv the measurements of weight and area thus amounts 

 to about ^ of a band, and is accordingly not much below the 

 limit marked out on the ground of the merely optical 

 examination. 



The conclusion is that some plates of mica are uniform in 

 an extraordinarily high degree and that there is perhaps no 

 reason for doubting that the thickness over finite areas may 

 be as uniform as is consistent with a molecular structure. 

 The stringency of the optical test might probably be increased 

 a few times by silvering the surfaces after the manner of 

 Fabry and Perot (compare Phil. Mag. xii. p, -189, 1906). 



* The mean thickness of the laver of grease would thus be about iX. 





H2 



