ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 851 



is claimed to be entirely different from any other form yet 

 made. 



It is constructed upon tlie principle of the " elb'ptical chuck," and 

 so enables either circles or ovals to be traced with equal ease. It 

 may also be used for cutting thin glass covers, either oval or circular, 

 as well as for general mounting purposes. 



Aylward's "Concentric" Turn-table.— This self-centering turn- 

 table, designed by Mr. H. P. Aylward, of Manchester, is shown in 

 Fig. 201. It is claimed to be accurately self-centering, and still 



Fig. 201. 



simple in its construction, so as not to be liable to get out of order, 

 and at the same time easy and rapid in use. It essentially consists 

 of two plates, the inner revolving on a pivot, whilst the outer one 

 revolves concentrically on the inner, some pins being so arranged 

 that by a simple turn of the outer ring they firmly grasp the slide 

 and centre it, a simple reverse movement instantly liberating it. It 

 will answer for slides of various widths, from 1 inch to 2^ inches. 



A is the ordinary wood block with steel pivot, on which the brass 

 revolving table turns. B, two brass springs which fit in holes in the 

 table, to be used when the slide is required to be out at centre (when 

 not in use they fit in holes in the block, as shown in the figure). 

 Hj brass ring which revolves concentrically on the table, and having 

 two conically headed pins J exactly 3^^^ of an inch apart, to allow 

 a 3 X 1 slide L to be placed lengthways between them. Two 

 similar pins F are so placed on the table, that upon revolving the 

 ring H they, in conjunction with the pins J, firmly grasp the oppo- 

 site corners of the slide, and cause the centre to coincide with the 

 centre of the table. I is a brass pin for more easily revolving the 

 ring on the table, and thus securing and liberating the slide. Non- 

 centering turn-tables can be converted into self-centering ones on 

 this plan at small cost. 



Photographing Bacteria.* — K. L. Kaschka writes that the excel- 

 lent photographs of Koch at first led him to believe that they could 

 be made without great difficulty, but an unexpected difficulty was met 

 with, so that it was impossible to find a colouring matter which 

 would so act upon the sensitive plate as to make the bacteria print 

 sufficiently black. Anilin colours, methyl- violet, fuchsin, and brown 

 were first tried, but only the first two coloured the bacteria well, and 

 although deeply coloured, as viewed by the eye-piece, they were scarcely 



* Zeitschr. f. Mikr., ii. (1880) pp. 264-5. See also Amer. Mon. Micr. Journ., 

 ii. (1881) pp. 53-4. 



