14 > Transactions of the 



III. — On an Optical Illusion Slide : Cracks in Silica Films. 



By Heney J. Slack, F.G.S., Sec. E.M.S. 



(^Eead before the Eoyal Microscopical Society, Nov. 9, 1870.) 



Although there may be much reason in the complaint of micro- 

 scopists that existing objectives leave them without the means of 

 distinguishing many characteristics of minute structure, such as 

 ultimate organic tissues, cell walls, &c., it is certain that the pro- 

 gress of knowledge is more often impeded by difficulties of inter- 

 preting what the best instruments can show. The long-continued 

 controversy whether the markings of diatoms consisted in elevations 

 or depressions, the discussions concerning the scales of Lepidoptera 

 and Thysanura, together with many others that will occur to the 

 practical student, are illustrations of this fact, and we can only 

 expect to arrive at greater success in discriminating between appear- 

 ances optically true under certain conditions, and veritable facts of 

 form and texture, by becoming famihar with the cu'cumstances 

 under which illusions arise. As a small contribution to this end, 

 the attention of microscopists is called to "An Optical Illusion 

 Shde," which may be thus prepared. Place a di'op of an aqueous 

 solution of colloid sihca, obtained by dialysis,* on a glass slide, and 

 evaporate it, either quickly over a lamp, or slowly in a place free 

 from dust. By this means a film of transparent silica is deposited 

 upon the glass, fissured in curious and compHcated patterns by 

 numerous cracks. These cracks when highly magnified present a 

 variety of forms, something like a veined leaf, or the patterns on a 

 complicated city map, with long Hues of straight and curved streets, 

 squares, ovals, circuses, &c., and in certain places all sorts of 

 angular and rounded outlines. In general the cracks take place 

 without materially altering the level of the surface, but occasionally 

 an edge will be found turned up. 



If a shde thus prepared be examined in difi'used dayhght, its 

 real character is not difficult to discern, especially if attention is paid 

 to the widest of the fissures and the largest of the isolated sihca 

 plates. By lamp-light, however, all the finer cracks, viewed as 

 transparent objects with a power of from 200 or 300 and upwards, 

 look like elevations, and that focussing which exhibits them in the 

 strongest relief would be supposed to be the most correct, from the 

 superior distinctness of its outhnes. 



By the employment of a central stop of an achromatic condenser, 

 the illusion becomes more striking. If a large angle of aperture is 

 used and the field flooded with hght, prismatic colours appear abun- 

 dantly, one edge of the crack being blue and the opposite one red, 



* Silicate of soda or potash is dissolved in distilled water, enough hydrochloric 

 acid carefully added to separate the alkali from the silica. The liquid is then 

 placed in a dialysing drum, and the crystalline salt escapes, leaving the silica dis- 

 solved in the water that remains. 



