620 SUMMARY OF CURRENT RESEARCHES RELATING TO 



case of field-glasses), were fixed adjuncts. As the use of binoculars for 

 histological purposes became obsolete, the advantage of the shade appears 

 to have been lost sight of. 



The drawtube of all Microscopes at the present time is made of a 

 standard inside diameter known as No. 1, namely 23 "3 mm., and this 

 is the size adopted for the aperture in the floor of the shade through 

 which the ocular drops ; but the aperture is made also of a larger size 

 to correspond with Standard No. 2, namely 26 mm., and it can of course 

 be cut so as to take an ocular of any other dimension. It is hardly 

 necessary to add that the shade can be used for either eye by rotating it 

 so that the higher part corresponds with the outer receding margin of 

 the orbit. The diagrammatic section, fig. 65 (natural size), shows the 

 shade in situ, as kept in position between the flat upper edge of the 

 draw-tube A and the rim at the top of the ocular. The form of the 

 upper opening of the shade is indicated by the dotted line. 



(6) Miscellaneous. 



Microscopical Characters of Volcanic Tuffs : a Study for 

 Students.* — L. V. Pirsson aims at the systematic treatment of the 

 characteristics of tuffs, which he classifies into (1) vitric, (2) crystal. 

 and (3) lithic tuffs. The subject is microscopically treated with the 

 view of elucidating the type features. But it must not be supposed 

 that all tuffs will clearly fall into one or other of these three classes. 

 While many will doubtless do so, the majority of these rocks will be 

 found to be intermediate in character ; for all gradations between the 

 three will be found in nature, with the exception that tuff's composed of 

 glass dust with stony ash particles, but devoid of individual mineral 

 crystals, must be extremely rare, if indeed they occur. The most com- 

 mon kinds are those containing in variable proportions all three ingre- 

 dients. Tuff's may also be fresh, altered, or metamorphosed, and the 

 author deals further with his subject from this point of view. 



B. Technique. t 

 (1) Collecting- Objects, including' Culture Processes. 



Bacterial Test for Plant Food Accessories (Auximones). J — 

 W. B. Bottomley has elaborated a bacterial test for plant food acces- 

 sories (" auximones "). These auximones are obtained from an alcoholic 

 extract of bacterized peat, the fractions of such extracts obtained by 

 means of phosphotungstic acid and by silver and baryta giving good 

 results with wheat plants. Cultures were obtained by placing 10 grin. 

 of garden soil in a flask containing 100 com. tap-water, 0*1 grin. 

 (NH 4 ) 2 S0 4 , 0-1 grm., K 2 HP0 4 , and 0*2 grin., MgC0 3 (Winogradsky's 



* Amer. Journ. Sci., xl. (Aug. 1915) pp. 191-211 (6 figs.). 



t This subdivision contains (1) Collecting Objects, including Culture Pro- 

 cesses ; (2) Preparing Objects ; (3) Cutting, including Embedding and Microtomes ; 

 (4) Staining and Injecting ; (5) Mounting, including slides, preservative fluids, etc. ; 

 (6) Miscellaneous. 



X Proc. Eoy. Soc, Series B, lxxxix. (1915) pp. 102-8. 



