ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 251 



MICROSCOPY. 



A. Instruments, Accessories, etc.* 

 (1) Stands. 



"Le Microscope," by Van Heurck (First Edition).! — A copy of 

 this scarce work has lately been presented to the Society, and its modest 

 dimensions are in strange contrast with the bulk of its latest representa- 

 tive. It is dated 1865, and consists of 108 pages, with thirty-four 

 figures in the text. Its purpose was to describe the Microscope as an 

 instrument (71 pp.), and then to discuss its application to vegetal 

 anatomy (33 pp.), a subject on which there was at that time no work in 

 French. Amongst the instruments is an ingenious tri-ocular, by Nachet, 

 intended for the simultaneous observation of an object by three 

 observers. The tubes are separated by angles of 120°, and the image 

 formed by the single objective is reflected by prisms through the tubes 

 to the eye-pieces. The instrument was set on a small circular table, 

 around which the observers sat at equal distances. 



(2) Eye-pieces and Objectives. 



Metrical Measures.^ — This title is scarcely appropriate to E. M. 

 Nelson's article, which is chiefly occupied with the structure of pre- 

 achromatic lenses and with their behaviour under certain conditions. 

 Many of the early lenses examined by the author possessed certain 

 excellencies, and their study revealed several interesting and useful 

 points. The author's general conclusion is that stops are hardly wanted 

 with low powers. With medium powers stops to 0*5 N.A. will be 

 found advantageous, and a ^-in. with a 0*65 N.A. stop and a B or C 

 eye-piece is a useful combination for pond hfe on a dark ground. 

 " Penetration " really means the improvement in an image owing to the 

 presence of a stop at the back lens of an indifferent objective. 



(3) Illuminating' and other Apparatus. 



Berget's Differential Refractometer for Measuring Sea-water 

 Salinity. § — Oceanographers require that the salinity of sea-water should 

 be known to a degree of precision marked by the fourth decimal place 

 and the half of the fifth. A. Berget has succeeded, by means of a 



* This subdivision contains (1) Stands : (2) Eye-pieces and Objectives ; (3) 

 Illuminating and other Apparatus ; (4) Photomicrography ; (5) Microscopical 

 Optics and Manipulation ; (6) Miscellaneous. 



t Published by Adrien Delahaye, 23 Rue de I'Ecole de Medicine, Paris. 



X English Mechanic, civ. No. 2694, Nov. 10, 1916. 



§ Comptes Rendus, clxiv. (1917) pp. 400-2. 



S 2 



