ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 335 



cavity and the correction of the back lenses ; secondly, the discovery 

 of suitable liquids ; and thirdly, the application of these liquids to 

 histological purposes. 



Imperfections in the cavity can scarcely be avoided, and will 

 with the secondary aberrations prove a source of some trouble to 

 opticians. 



As regards the liquids to be used, methylene-iodide appears to be 

 the best at present available ; it becomes brown in the light, owing to 

 evolution of iodine, but the colour may be removed by shaking it 

 with aqueous potash-solution; it can be applied directly to dry 

 diatoms, but histological preparations must be previously treated 

 with absolute alcohol and monobromide of napthaline. More suitable 

 liquids may yet be discovered. 



With regard to the third point, the cover-glass must be dispensed 

 with, and the immersion liquid used in contact with the object ; this, 

 however, introduces no innovation for histological preparations, and 

 has this advantage that the principle of homogeneous immersion 

 suffers no disturbance when the cover-glass is abolished. To get rid 

 of difficulties arising from differences of refractive power in the 

 tissues and the immersion-liquid it will be necessary to increase the 

 cone of illuminating light as far as possible; with a cone of 180° 

 this difference would theoretically be eliminated. Abbe's illuminator 

 is not sufficient, and the best plan is to use thin plates of white glass 

 as object-carriers, and illuminate them brilliantly from beneath. 



The cavity in the front lens might be dispensed with if the crown- 

 glass, meniscus could be replaced by a flatter plano-convex lens of 

 diamond ; unfortunately it is not possible to give any considerable 

 curvature to the diamond. If the diamond lens could be used many 

 advantages would be gained even with oil of low refractive power, 

 e. g. with an immersion liquid of index 1 • 5 it would only be necessary 

 to correct for 60° instead of 120° as is the case at present with the 

 most powerful objectives ; and in addition to easier correction a larger 

 aperture would be obtained. 



The Aperture Question. — We were not a little surprised to 

 receive lately an elaborate discussion on Aperture and Microscopical 

 Vision, written in Spanish, which we should have supposed to be one 

 of the most unlikely languages of Western Europe in which such a 

 subject would be treated of. It is from the pen of Don Joaquin 

 M*- de Castellarnau y de Lleopart,* who in other papers previously 

 published has shown himself to be much in advance of the majority 

 of his countrymen in a knowledge and appreciation of both theoretical 

 and practical microscopy. 



The present work is extremely well put together ; indeed, it is 

 quite unique in the completeness of its treatment of the question. 

 If there now remained in this country any microscopists who seriously 

 questioned either the fact of an aperture in excess of 180° in air, or 



* ' Vision Microscopica. Notas sobre las Condiciones de Verdad de la Imagen 

 microscopica y el niodo de expresarlas.' 96 pp., 1 pi., and 3 figs, 8vo, Madrid, 

 1885 (sep. repr. from Anal, de la Soc. Esp. de Hist. Nat., xiv. (1885) pp. 257-352. 



