ZOOLOGY A.ND BOTANY, MICROSCOPY, ETC. 821 



and central sliding-board are mounted, so that the camera, Microscope, and 

 lantern can be raised to a convenient height above the ground. 



The various parts of the apparatus are described more in detail as 

 follows : — The Microscope utilized was one constructed by Zeiss, but any 

 good stand may be adapted in the same way. The advantage of Zeiss's 

 stand, for bacteriological photography, is that the wide stage forms a steady 

 support for cultivations on small panes of glass coated with nutrient jelly, 

 A mechanical stage greatly facilitates manipulation with the highest powers ; 

 but it is not indispensable, for Dr. Crookshank has taken, without the use 

 of one, a large number of photograi>hs, though employing, as a rule, a 

 1/25 hom. imm. It is most essential that the Microscope should be 

 perfectly steady. To ensure this the horseshoe foot-piece of the Zeiss 

 stand fits under a projecting ledge, and is then clamped by a cross-piece, so 

 that it is firmly fixed. 



The Microscope, with the means for clamping it, and the oxyhydrogen 

 lantern are carried upon an independent sliding-board, which admits of 

 movement to or from the camera. The sliding-board also moves upon a 

 centre, which enables the Microscope to be turned out from the median 

 line ; in fact, to be turned at a right angle to the position it occupies when 

 ready for the exposure. The object of this contrivance is to enable the 

 operator to sit down by the side of the apparatus, and with comfort to 

 arrange the object in the field of the Microscope. On turning tlie Micro- 

 scope back into the median line, it is fixed in the optical axis of the 

 apparatus by means of a stop. The sliding-board was originally provided 

 with a small grooved wheel receiving an endless cord, made of silk or 

 fishing-line, which passed round the grooved, milled head of the fine 

 adjustment. When the sliding-board was returned to the median line of 

 the apparatus, the milled wheel connected with the fine-adjustment im- 

 pinged upon the wheel of the long focusing rod. The latter was provided 

 with an indiarubber tyre, which grip2)ed the teeth of the milled wheel, and 

 thus the long focusing rod was placed in connection with the fine-adjust- 

 ment. Dr. Crookshank now dispenses with this arrangement, as he believes 

 it to be a mistake to strain the objective by having the screen at a greater 

 distance from the object than, say, 30 inches, and with that distance of 

 screen one can easily move the fine-adjustment with one hand, while holding 

 the focusing glass in the other. 



Of equal importance to the objective is the sub-stage condenser, and 

 this, for the best results, must be provided with arrangements for focusing 

 and accurate centering. 



For illumination the author has chiefly employed the oxyhydrogen light, 

 which can be used without the interposition of a mirror in either position 

 of the apparatus. In the horizontal position a paraffin lamp may be 

 employed by simply removing the lantern and substituting the one for the 

 other ; but to employ this illumination when the apparatus is vertical 

 would obviously entail another arrangement. It would in this case be 

 necessary to adjust the mirror of the Microscope and to place the lamp in 

 such a position that the light would be reflected in the ordinary way. 



If the paraffin lamp be preferred, it should be provided with a large 

 broad wick and a metal chimney. The burner may he made to revolve, so 

 that either the edge or the flat of the flame may be utilized. The metal 

 chimney has an aperture in front, giving exit to the rays of light, which is 

 closed in by a slip of glass. The glass is very liable to crack when exposed to 

 the full force of the flame, and it is as well, therefore, to be provided with 

 a stock of glass slips, which have been annealed by being enveloped in a 

 cloth and boiled for two or three hours. 



