INVERTEBKATAj CRYPTOGAMIA, MICROSCOPY, ETC. 951 



tben moved along till the first line of the second docimetro forms a 

 contact with the zero of tho eye-piece micrometer, wlien at the same 

 time contact is formed with the first stoj). Moving tho plate to the 

 second stop the reading for the second decimetre is taken. A com- 

 parison of the several values obtained with the mean value will 

 show how much each is in error, provided the entire length is 

 correct. 



Tolles' -V Objective. — Dr. Cutter gives some further particulars 

 respecting this objective (made in 1873), in the ' Journal de Micro- 

 graphie.'* It works both dry and immersion, is composed of three 

 systems, and has 170^ aperture. Its frontal distance is tt^o inch. Tho 

 correction collar moves only I of a circle. The aperture of the front 

 lens is -f}:^ inch, and the " diameter of the objective at the other ex- 

 tremity " i inch, its length being about 2h inch. The field is remark- 

 ably clear and very flat, the resolution good, and the definition, having 

 regard to the enormous amplification, excellent. 



Eezner's Mechanical Finger.f — This form was designed by Dr. 

 W. B. Eezner, of Cleveland, Ohio, and is adapted to any Microscope, 

 whereas the forms heretofore made were only designed for Micro- 

 scopes having substages. 



Fig. 2. 



In use, the sleeve, seen in Fig. 2, is passed up over the objective 

 far enough to have firm bearing, and so that the bristle point will be 

 in focus when depressed nearly to its limit ; it is clamped in place by 

 the small thumb-screw. Tho wire in which the bristle is carried is 

 drilled at the point to receive it, and slides easily, but not loosely, 

 through a small sleeve, so that the end of the bristle can be brought 

 into the centre of the field when in focus, and the wire can be revolved 

 so as to view every side of the object picked up by the bristle. The 

 wire stands at a greater angle than is shown in the cut, and the 

 vertical part of the spring is not so long as figured. 



When using the finger, the bristle is first raised by means of the 

 micrometer screw till so far within focus as to be nearly or quite 

 invisible, then the objective is focussed on the slide and the desired 

 object sought for and brought to the centre of the field ; the bristle 

 point is then lowered by the screw till it touches the object, which 



* ' Journ. de Micr.,' iii. p. 207. 



t ' Am. Journ. Blicr.,' iv. (1S710 p. 05. 



