42 HOW TO WORK 



of the lever D, the longer one / has a hole to allow the screw h to 

 pass without touching it ; g, a screw holding the two springs and the 

 elastic arm to the arm of the cap ; //, a milled screw to adjust the 

 elastic arm C, so as to bring the diamond point away from the centre, 

 arrording to the size of the ring required. When the object has 

 been found, the cap carrying the diamond is placed on the object- 

 glass and carefully adjusted, so that the diamond point is brought 

 into contact with the surface of the glass, it is then turned round, 

 and thus a line is drawn round any object which can be readily found 

 at any future time. 



This same end has been gained in another manner. Graduated 

 scales have been affixed to the stage of the microscope, so as to 

 measure the exact amount of movement in the vertical and horizontal 

 direction ; the slide being placed in position against a stop at the 

 side. The number on the two scales is noted when the object is 

 seen in the field, and, by placing the stage opposite the same numbers, 

 at any future time the object must appear in the same position. 

 Various ingenious " finders " have been proposed. A very simple 

 and efficient one is represented in pi. XIII, fig. 62, in which the 

 scales are ruled on paper (Mr. Wright, Microscopical Journal, vol. I, 

 p. 302, 1853), which is afterwards fixed upon the stage. It is better 

 to have the lines ruled on the brass itself. 



Bailey's Universal Indicator. Mr. J. W T . Bailey, of the United 

 States, has described an instrument for registering the positions of 

 various objects upon a slide, in vol. IV of the Quarterly Journal of 

 Microscopical Science. This indicator is to be firmly fixed to the 

 stage of the microscope, care being taken that the centre of the 

 indicator corresponds to the centre of the object-glass. The mode 

 of using the indicator is obvious. 



All such devices have, however, been superseded in cases where 

 the microscope is provided with a travelling stage, by the two follow- 

 ing very clever arrangements, the first suggested by Mr. Maltwood 

 (Trans. Microscopical Society, vol. VI, p. 59, 1858), the second by 

 Mr. Bridgman, of Norwich. In order to use Maltwood's finder, a 

 little stop is placed upon one side of the stage, in contact with which 

 one end of the finder, and afterwards the glass slide containing the 

 object can be placed. The finder consists of a plate of glass, upon 

 which numbers are arranged in minute squares. These run in two 

 directions, vertically and horizontally, so that in each square there 

 are two different numbers, except in the case of the central square, 

 which of course contains two 255. Any object having been found, 

 its exact position may be registered by removing the slide and placing 

 on the stage the finder. The numbers seen in the field are then 



