Mi PROCEEDINGS OF THE 



a X 15 for a short. The term " optical index," wldch is the 

 N.A. X 1000 -^ initial magnifying power, was introduced, 

 and objectives to conform to the plan must have an optical 

 index of 25 or N.A, '25 for each initial magnifying power 

 of 10, Mr. Nelson proposed to bring before the Club a power- 

 aperture curve based on his own experience. A series of curves 

 was then shown on the screen, the vertical scale represent- 

 ing power and the horizontal aperture. Mr. Nelson advised 

 beginners to keep to one series of objectives. Low powers should 

 be used when possible, and it is a great mistake to rush after 

 high powers. In conclusion, Mr. Nelson advocated large fields to 

 facihtate finding objects, and pointed out that in measuring 

 magnification it is more reasonable to consider the power of the 

 eyepiece as fixed, and the power of the objective as varying 

 accordingly to the tube length than vice versa. 



The Hon. Secretary then read " Notes on Mounting in Sandarac 

 and other Media," by Mr. W. M. Bale, of Melbourne. Mr. Bale's 

 experiments were suggested by Mr. T. E. Wallis's paper, Q.M.C. 

 Journal, April 1919. Mr. Bale finds amyl-sandarac medium good 

 for mounting radulae of mollusca. He prefers to use a thickened 

 solution to obviate the need for adding more after mounting to 

 replace loss by evaporation, and when there is no danger of 

 shrinkage mounts direct from methylated spirit. He finds the 

 substitution of methylated spirit (which Mr. Wallis condemns) 

 in place of amyl-alcohol as a solvent for the sandarac satisfactory 

 in use. He has experimented with gum thus (frankincense) 

 as a mounting medium. Its only advantages over Canada 

 balsam are its perfect solubility in alcohol and its lighter colour, 

 but there seems to be some possibiUty of the formation of 

 crystals. Mr. Bale then dealt with the sealing of fluid mounts. 

 He strongly recommends the use of a cover larger than the cell. 

 The cover being held in place by a tight clip, if necessary, while the 

 excess of mountant is gently squeezed out, and the space between 

 the edge of the cover and the slip having been carefully syringed 

 out and allowed to dry, gold size or other sealing cement is run in. 

 Linseed oil, which hardens very slowly, may be used as a mountant 

 instead of castor oil for crystals, etc. Mr. T. E. Wallis, in com- 

 menting on Mr. Bale's communication, said that he preferred 

 amyl-alcohol as a solvent for sandarac because, on evaporation, a 

 homogeneous resin remained, whereas, if it was dissolved in 



