60 BAILEY, ON A UNIVERSAL 



increased by several slight changes. One of these consists in 

 removing the paper centre-piece, and replacing it either tem- 

 porarily or permanently by a glass plate bearing lines at right 

 angles to each other ruled very lightly with a diamond point, 

 and so adjusted as to coincide with the prolongation of C D 

 and E F through the centre. For all but the highest powers 

 there is no objection to having these excessively minute lines 

 permanently beneath the centre of the Indicator, as they do 

 not perceptibly interfere with the light, and it is convenient 

 to have them always in place. They can be ruled upon a 

 piece of mica or thin glass cemented to the back of the 

 Indicator, or the latter may itself be cemented to a piece of 

 plate glass and the central guide-lines then carefully ruled. 

 Even for the highest powers these lines can be used in record- 

 ing the position of objects, which can then be found for study 

 by using an Indicator of the ordinary form. By a proper 

 arrangement, a moveable stage, with screws for vertical and 

 horizontal motions, inay be graduated so as to correspond to 

 the Indicator, and yet preserve all the advantages of accurate 

 adjustment which the screws afford. For this purpose it is 

 necessary to observe that if the Indicator be placed upon the 

 stage and accurately centred, with its guide-line, C D, parallel 

 to the front edge of the stage, and a slide be then placed upon 

 the Indicator, so that its horizontal guide-line shall coincide 

 with C D, and the right-hand vertical guide-line stand at 70, 

 (^, e. in the position which would be recorded as 7 S'j) or its left- 

 hand guide-line at | g' ; then a motion of the stage itself bearing 

 with it the Indicator and slide, or an equal motion of the slide 

 upon the Indicator and fixed stage, will bring the same point 

 of the slide to the centre of the field of view. 



Therefore, by attaching to the stage in any convenient 

 manner graduations corresponding to those of the Indicator, 

 and by having lines corresponding to f S' and \l' ruled upon 

 the stage, it will only be necessary to place the slide directly 

 on the stage at these numbers, the stage itself being set either 

 at f §' or ^^ of its graduations. By turning the milled heads 

 of the screws which give the vertical and horizontal motions 

 of the stage, the object can be brought into the field of view, 

 and recorded or found again by means of the numbers attached 

 to the stage ; while the record may be used for any other 

 Indicator as if made in the usual manner. If the distance 

 between the guide-lines upon the slide agrees accurately with 

 that between 40 and 70 of the Indicator, the slide, when placed 

 upon the moveable stage at either fg' or f g', will need no 

 displacement for the whole series of numbers ; but if this 

 distance do not agree, the slide must be put with its left- 



