Royal Microscopical Society. 213 



HI. — 0]}tical Appearances of Cut Lines in Glass. 

 By Henry J. Slack, F.G.S., Sec. K M.S. 



(^Read he/ore the Eoyal Microscopical Society, Ajml 5, 1871.) 



The use of higli powers in delicate investigations renders it neces- 

 sary that the microscopist should study the character of appearances 

 which arise from optical laws, and which can only be rightly inter- 

 preted by referring them to forms and structures to which they bear 

 no real or exact resemblance. A short time since the writer called 

 the attention of the Society to the deceptive nature of the appear- 

 ances presented by fine cracks in silica films ; and further observa- 

 tions show that if the finest or narrowest of such marks are selected 

 for examination, the chances of obtaining perfect illusions are in- 

 creased by the amount of magnification and the perfection of the 

 objectives employed. Dehcate interference bands, pseudo-beading, 

 &c., look more real with well-corrected object-glasses than with bad ; 

 and careful illumination will often add to the structural aspect of 

 mere optical efiects. 



The edges of silica cracks, obtained as mentioned in a former 

 paper, difier from the edges of minute furrows cut in glass, by being 

 smooth instead of jagged. The latter as well as the former are well 

 worth study. Preparatory to examining such furrows as are cut 

 with diamonds in glass for micrometers or diflraction gratings, it is 

 well to notice the edges of thin glass cut for slide-covers. If half- 

 a-dozen or more thin glass squares are held close together,* and 

 viewed, edges upward, as transparent objects, a variety of curious 

 optical efiects will be seen, arising from interfering reflexions and 

 refractions. The examination should begin with an inch or frds, 

 after which | inch, and ^ or -^th will be advantageously employed. 

 It is easy to focus parts of the glasses' edges, so as to show their 

 true form ; but portions a little in or out of focus will show beads, 

 appearances like columns of Egyptian architecture, &c. Most of 

 these optical appearances are sufficiently hazy or confused to give 

 warning of their true nature; but generally some will be found 

 so sharp and clear that, if viewed separately, they might easily 

 mislead a practised observer. In making these experiments, it is 

 best to have handy a box containing at least several dozens of the 

 thin glasses, as some sets will prove much more interesting than 

 others. They should be viewed with their edges parallel to the 

 plane of the objective, and also at various angles. The corners of 

 the squares should also be looked at. 



Lines cut in glass for micrometers or difiraction-gratings are 



* The stage vice made for me by the late Mr. Thomas Ross is very haudy for 

 this and many other pm-poses, as the blades approach each other in a parallel 

 direction, and can be lixed at any distance with great nicety. 



