244 SUMMARY OF CURKENT RESEARCHES RELATING TO 



occur. The usual type of halo, as seen in rock sections, is a dark spot 

 of roughly circular outline surrounding a small centrally situated inclo- 

 sure in another mineral which itself may, or may not, also be pleochroic. 

 The author has found that the following minerals usually show halos : — 

 Beotite, augite, hornblende, mnscovite, chlorite, tourmaline, cordierite, 

 staurolite, and audalusite. There is sometimes a difficulty in the 

 identification of the mineral producing the halo, but the author has 

 detected zircon, sphene, apatite, orthite (allanite), and epidote. All 

 these latter minerals (except, perhaps, epidote) are well known to be, 

 comparatively speaking, strongly radio-active. As far as the rocks are 

 concerned, halos are far more common in those of igneous origin than 

 in the other classes, and are especially noticeable in the plutonic types, 

 particularly the granites. The halos are usually spherical in shape, but 

 irregular grains, or granular aggregates, produce halos of corresponding 

 shape, the coloured margins being, however, of uniform width. This 

 uniformity of width is a remarkable feature, the measurement of a large 

 number of cases giving few variations from ■ 03 mm. Professor Joly 

 has pointed out that the penetration of the a rays emitted by radium 

 compounds is about * 04 mm. in the case of aluminium, and, having 

 regard to the slightly greater density of the minerals examined, the 

 results are in close agreement with the theory that the halos are due to 

 the alteration of the surrounding minerals by these rays. 



(6) Miscellaneous. 



Homogeneity of Optical Glass.* — W. Zschokke points out the diffi- 

 culties in producing homogeneous glass. The importance of the subject 

 needs no demonstration, but the attainment of homogeneity seems im- 

 possible. Even the best compounded and cooled glass-meltings vary 

 considerably in their refractive index. The variation would be less im- 

 portant if the manufacturer had only to make a single lens, but his task 

 is more frequently the manufacture of compound lenses and of reproduc- 

 tions. As a means of testing want of homogeneity, the author suggests 

 the cutting of a right-angled prism from a given slab. By telescopic 

 observations on an " infinitely " distant object seen through the prism, 

 the refractive index can be calculated for different parts of the prism. 

 The knowledge thus obtained may be useful in selecting a suitable part 

 for lens manufacture. 



Spiers' " Nature through the Microscope." f — This work, the sub- 

 title of which is " The Bambles and Studies of a Microscopist," is a 

 popular account of some of the better-known " Marvels of the Micro- 

 scope." It is written in language as simple as the subject-matter permits, 

 and the descriptions convey as much information as a quite uninstructed 

 observer may be expected to assimilate. It is designed primarily to 

 interest such an observer in the Microscope and its revelations, and 

 also to assist a beginner in the choice and use of an instrument. The 

 volume is copiously and satisfactorily illustrated. 



* Zeit. f. Instrumentenk., xxix. (1909) pp. 286- 9 (1 fig.). 



t London : Culley (undated) 335 pp. (10 col. pis. and about 300 drawings). 



