ZOOLOGY AND 1SOTANY, MICROSCOPY, ETC. 587 



water ; from this generator it is passed into a gas-holder. When the 

 gas is used up the gas-holder falls, and this action causes another charge 

 of carbide to drop into the water. The generator should have two gas 

 mains, one of 12-in. pressure for lighting and incubator purposes, and 

 another of 4-in. pressure for Bunsen burners and gas stoves. 



Engelmann, Th. W. — Ueber ein Mikrospektralobjectiv mit Normalspectrum. 



Arch-, f. Anat. u. Physiol., l'.HIO, Physiol. Abth. Supp., p- 338. 



Weixschenk, Dr. Ernst — Anleitung- zum Gebrauch des Polarisationsmikro- 

 skops. (Guide to the use of the Polarising Microscope.) 



[The author seems to have written a very thorough and valuable treatise.] 

 Herder (Freiburg im Breisgau), 123 pp. and 100 figs. 



(4) Photomicrography. 



Instantaneous Photography of Growing Crystals. * — Th. W. 

 Richards and E. H. Archibald, who have studied the growth of crystals 

 by means of instantaneous photography, used Bausch and Lomb's outfit. 

 Between tbe Microscope and camera was placed, in a light-tight box, a 

 revolving shutter which allowed an exposure of a tenth of a second. A 

 Henrici hot-air motor, combined with speed-reducing double pulleys, 

 enabled the experimenter to use any rate of revolution desired. Two 

 different arrangements were adopted, the first of which caused the suc- 

 cessive impression of a bright image in a dark field, and the second 

 registered dark images in a succession of bright fields. In carrying 

 out tbe first of these two methods it was found more convenient to move 

 the crystallising solution than to move the photographic plate. By the 

 foregoing methods it was found possible to take very frequent photo- 

 micrographs of crystals during their birth and growth. An enlarge- 

 ment of over four thousand diameters was obtained, and both common 

 and polarised light were used. Only substances with high melting 

 points were examined, and the crystallisation was always from aqueous 

 solution. No properly focussed image in any of the plates seemed to be 

 devoid of crystalline structure. The growth in diameter during the first 

 second of the crystal's life was found to be vastly greater than during 

 the subsequent period. The authors' observations do not lend support 

 to the notion that crystals develop from a transitory liquid phase. 



Sanger, Shepherd, & Co.'s Process of Natural Colour Photo- 

 graphy. — Fig. 118 represents the apparatus used by Messrs. Sanger, 

 Shepherd, & Co. for colour-record negatives of any object. The appa- 

 ratus known as a repeating-back is attached to any front-focussing 

 quarter, half, or whole-plate camera by means of a panel cut to corre- 

 spond, and interchangeable with the ordinary dark slide. This does 

 not interfere with the use of the camera for ordinary work, while it 

 allows of the attachment being adjusted for use in a few seconds. An 

 outer case (on left of figure) carries the frame with the three colour 

 filters and the double dark slide holding two spectrum plates. The 

 colour-hlter frame is furnished with lugs which engage with the dark 

 slide so that both move together. The first exposure through the red 

 filter is made with the filter-frame and dark slide at the right-hand end 



* Proc. Amer. Acad. Arts and Sci., xxxvi. (1901) pp. 341-53 (3 pis.). 



