742 SUMMARY OF CURRENT RESEARCHES RELATING TO. 



tendency to radiating arrangement. Among the substances inclosed 

 were certain vacuolated masses, exactly similar to the drops of liquid 

 found in crystals; they were very small, the vacuoles being very 

 wide with dark margins. No movement was ever observed in the 

 latter, but an argument in favour of their liquid nature is that as 

 soon as (by melting) they are at the margin of the section of hailstone, 

 they suddenly become empty, while the surrounding ice persists for a 

 time. The exact nature of the other dirty and dust-like masses 

 inclosed, which were by no means scarce, could not be determined. 



Appearances presented by Air-bubbles and Fat-globules in 

 White and Monochromatic Light. — We extract from Professor 

 Eanvier's work on Histology,* the figures which illustrate the appear- 

 ance at various points of the focus of an air-bubble in water and 

 Canada balsam, and of a fat-globule in water, a diaphragm of about 

 § of a mm. being placed at a distance of 5 mm. beneath the stage, 

 and the concave mirror exactly centered. 



Air-bubbles in water. — Fig. 142, No. 1, represents the different 

 appearances of an air-bubble in water. On focussing the objective to 

 the middle of the bubble (B), the centre of the image is seen to be 

 very bright, brighter than the rest of the field. It is surrounded by 

 a greyish zone, and a somewhat broad black ring interrupted by one 

 or more brighter circles. Eound the black ring are again one or 

 more concentric circles (of diffraction) brighter than the field. 



On focussing to the bottom of the bubble (A), the central white 

 circle diminishes and becomes brighter, its margin is sharper, and it 

 is surrounded by a very broad black ring, which has on its periphery 

 one or more diffraction circles. 



When the objective is focussed to the upper surface of the bubble 

 (C) the central circle increases in size, and is surrounded by a greater 

 or less number of rings of various shades of grey, around which is 

 again found a black ring, but narrower than those in the previous 

 positions of the objective (A and B). The outer circles of diffraction 

 are also much more numerous. 



Professor Eanvier explains these appearances by reference to 

 Fig. 143, which is a sectional view of an air-bubble (in water) 

 receiving upon its base a series of parallel rays. The rays which 

 passes through the centre of the bubble (undergoing no deviation) 

 and those at a, a', a" (which are more or less deflected by refraction) 

 reach the eye of the observer, whilst a" being incident at the 

 limiting angle for rays which pass from water to air (48° 35') is 

 totally reflected, and does not reach the eye. The same is the case 

 with the rays beyond a'", so that the margin of the bubble has a dark 

 zone, varying as in Fig. 142, No. 1, A, B, C, according as the objective 

 is focussed to the lower, central, or upper parts. 



Air-bubbles in Canada Balsam. — Canada balsam being of a higher 

 refractive index than water, the limiting angle instead of being 

 48° 35' is 41° only, so that rays which are incident much less obliquely 

 on the surface of separation undergo total reflection, and it will be 



* Traite technique d'Histologie, 1878, pp. 14-20 (4 figs.). 



