Royal Microscopical Society. 15 



aud in many places rows of narrow coloured bands like Newton's 

 rings will be noticed, especially if the power is high enough — say 

 600 to 800 and upwards. Unilateral light does not clear up the 

 question, an eye being still best satisfied with the focussing that brings 

 the lines of pseudo-elevation into the greatest sharpness and relief. 



With dark-ground illumination a very beautiful effect is pro- 

 duced ; the edges of the cracked silica film being brilhantly lumi- 

 nous, while the uncracked portion of the film, and the interspaces 

 between the luminous edges of the cracks are dark. The larger 

 portions of the fractured silica, and those most detached, show the 

 true nature of the object with this illumination ; but the same can 

 scarcely be said of the finer cracks, which might easily be taken for 

 small tubular vessels anastomosing in all directions. 



In these experiments, the eye is most readily deceived by a high 

 power. For example, let the slide be viewed with a ^-inch and an 

 A eye-piece, with transparent illumination ; the true nature of many 

 cracks will be seen ; then substitute a C or D eye-piece, and the eye 

 becomes most doubtful of that focussing which is nearest the truth ; 

 the aspect of bright clear ridges being most satisfying and agreeable. 

 The reason of this is evident. When the surfaces of the silica films 

 are focussed, there is nothing particular to see, as they are smooth, 

 clean, and exceedingly transparent, while the cracks look dingy and 

 imperfectly defined. A little lower focussing does not seem wrong, 

 because there is nothing on the silica surface to become indistinct, 

 while the cracks appear in fine relief, rounded and often slightly 

 tinged with colour from refractions, or reflexions, or both combined. 



A l-inch or xVinch Lieberkuhn reveals sliglit inequalities in 

 the surface of the split films, and when they are illuminated with 

 Messrs. Powell and Lealand's modification of Professor Smith's illu- 

 ininator for opaque objects with high powers, many fragments of the 

 film exhibit prismatic bands, and some show complete series of New- 

 ton's rings. It is necessary to use the smallest of the diaphragm 

 stops with this apparatus, or the field will be flooded with misty light. 

 The larger fragments give the best colours, which seem to arise from 

 the tension of the silica in drying, leaving minute air-spaces of vary- 

 ing thicknesses between the glass side and the films. The coloured 

 bands vary in shape from approximate circles and ovals to other 

 curves of various characters. The circular form is best shown where 

 the film has solidified round a central boss. 



Portions of the films sometimes appear of the same dark-grey 

 tint of the ground, while others are luminous. This appears to 

 result from the obscure parts being in such complete contact with 

 the glass, and with upper surfaces so completely parallel to the glass 

 surface as to produce the same effect upon the light which the glass 

 surface does. 



VOL. V. G 



