ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 721 



bination shutter and view-tube are made to be clamped by means of 

 three thumb-screws to the draw-tube of the Microscope ; this apparatus 

 (fig. 194) is fastened on after the ocular has been inserted in the draw- 

 tube. The details will be understood from the following description. 

 Upon a movable brass plate inside a light-tight box, placed just below 

 the camera bellows, is a 90° glass prism mounted in such a way that all 

 of the light which passes through the Microscope is projected upon a 

 piece of ground glass at the end of a cone, which may be lengthened 

 or shortened in order to give correct focus to the object, when it is 

 properly focussed upon the ground glass of the camera directly above 

 the Microscope. Next to the prism is a hole in the brass plate for 

 allowing light to pass from the Microscope directly to the photographic 

 plate, when the prism is moved by means of a spring and pneumatic 

 release ; finally there is a sufficient area of the brass plate to cover the 

 opening when exposure has been made. To take a photograph, the 

 microscopic animal is placed in a drop of Water upon a suitable glass 

 plate, the light is turned on, and the shutter so set that the object may 

 be focussed upon the ground glass of the cone. The plate-holder is 

 inserted and the dark slide drawn, leaving the plate exposed inside the 

 camera bellows. The movements of the animal are easily seen upon 

 the ground glass, and when the desired position is obtained the shutter 

 is released, the prism moves out of the way, and the light passes to the 

 plate. Although the apparatus has not reached the perfection desired 

 by the inventor, he has had satisfactory results with exposures as short 

 as 1/40 of a second, and considers that 1/100 of a second is attainable 

 with low-power objectives. The magnification has ranged up to 200 

 diameters. 



(5) Microscopical Optics and Manipulation. 



Zeiss' Immersion Refractometer. — This instrument (figs. 195, 196) 

 is intended for the examination of liquids of low refractive index, such 

 as dilute solutions. The scale value ranges from 1*325 to 1-367. Its 

 nature is that of a special form of Abbe's refractometer, the outer glass 

 prism (which transmits the light to the fluid layer) being made remov- 

 able, so that the lower end of the instrument may be dipped into the 

 fluid to be examined. At the same time, by means of a special trough, 

 or by careful hand motion, the operator finds a position by which the 

 incident ray suffers total reflection. The advantage is that the refractive 

 index is ascertained with the same ease as its temperature with a ther- 

 mometer or its specific gravity with a hydrometer. But even more 

 useful is the fact that the removal of the prism causes a much brighter 

 definition of the emergent ray than that from a mere layer of fluid be- 

 tween Abbe's two prisms ; a greater telescope magnification becomes 

 therefore possible, as well as a corresponding accuracy of measurement. 

 The hand telescope is of about 10 diameters magnifying power; the 

 prism P is of 63° refractive angle and made of hard glass of refractive 

 index 1-51. The apparatus is so adjusted that at a temperature of 

 17-5 C, the line for distilled water falls on division 15*0 of the scale. 

 In order to measure the line for any other liquid a micrometer Z is 

 fitted in the ocular. The floor of the trough A is furnished with a 

 suitable window. A pair of Amici prisms at A (fig. 198) neutralise 

 chromatism. 



