INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 623 



template till the centre is brought to the focus ; the cavity is then 

 finished in the same way as a concave lens. Finally, while rotating 

 in the lathe the paraboloid is perforated through the axis with a steel 

 drill and turpentine. 



Paraboloids can be made true enough for most purposes if finished 

 as above described ; but if great accuracy is a desideratum, the figure 

 may be corrected after the rough turning by means of the following 

 appliance. 



It is a property of the paraboloid that the face of every section 

 taken parallel to the axis, is an exact counterpart, and, in form, is the 

 same parabola. This enables us to verify and correct the figure. 

 From the further end of a base board, clamped to the bed of the lathe, 

 hinge a piece of board about two inches wide. Let this be so adjusted 

 that when the front edge is raised, the upper plane of the board falls 

 exactly parallel with the lathe centres. Rough file out a piece of sheet 

 brass, something like the template, to serve as a grinder. Lay this 

 on the face of the hinged piece of wood, and press it up on the revolv- 

 ing glass, smeared with fine emery and water. After a few turns, 

 lower the board and shift the brass grinder endways to another posi- 

 tion, either in or out. Eepeat this continually, occasionally turning 

 the brass over to equalize the sides. By this operation the parabolic 

 figures, of both the grinder and the glass, will soon correct each other. 

 Of course a piece of the swing board must be scooped out sufficiently 

 to admit nearly half the paraboloid. 



In accordance with the above mode of procedure, the parabola is 

 originated and its size predetermined by the given focal distance. 

 The ordinary dry parabolic illuminator is usually made about 1 inch 

 focus ; for an immersion yL will do better ; but if this is to be used as 

 an animalcule-holder, ^^ will be found sufficient. 



Black-Ground Illumination. — Mr. Wenham in the same article 

 says, that he has never seen minute organisms or animalcula so beau- 

 tifully displayed as by the truncated paraboloid described in a paper 

 published in the ' Transactions of the Microscopical Society of London ' 

 in 1856. The paraboloid is mounted so as to be used also as an 

 animalculse cage or live box, the object to be placed in water on the 

 flat top, and confined by a thin glass cover ; rays from a lamp made 

 parallel to be sent in beneath, using a dry object-glass. The minutest 

 details are visible in their true colours on a black field. He is not 

 aware that anyone has provided himself with this piece of apparatus, 

 and the knowledge of its effects probably does not extend beyond the 

 half-dozen friends who have seen his demonstration. 



Rotating Clips for Cheap Microscopes. *^ — The rotating stage, 

 when well made, is acknowledged to be a most imj)ortant and useful 

 addition to any Microscope. It has this objection, however, that 

 when well made it is expensive, and when badly made it is worthless. 

 Moreover, it adds considerably to the thickness of the stage, thus 

 interfering with the use of very oblique light. 



* ' Am. Journ. Micr.,' iv. (1879) p. 93. 



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