58 BAILEY, ON A UNIVERSAL 



believed that the explanation above given is sufficiently ex- 

 plicit to enable any one to use the Indicator ; but some addi- 

 tional remarks will now be made upon the reasons for adopting 

 the particular arrangement I have used, the modes of insuring 

 accuracy in manipulation, and the modifications of which the 

 Indicator is susceptible for individual convenience. 



It was desired to make the instrument capable of universal 

 application, so simple that it could be adapted to any stage ; 

 so light and yet so strong that it could be sent without injury 

 by mail or otherwise to any distance ; and, lastly, that the 

 different copies should be perfect fac-similes of each other 

 and reproducible at any time. All this is secured by having 

 the Indicator engraved upon a steel plate and printed upon 

 cards of uniform quality, and by taking the dimensions from 

 the standard United States inch, preserved in the office of the 

 Superintendent of Weights and Measures in Albany. In order 

 to extend the use of the Indicator to all cases which are likely 

 to occur, the graduation was arranged with reference to slides 

 three inches long and one inch wide, while it will answer 

 equally well for smaller ones. When these slides are not 

 covered with paper, and guide-lines can be ruled as above 

 directed upon the glass itself, the graduations necessary for 

 their use would only extend upon the verticals ^ an inch 

 above and below the horizontal line, and upon the horizontal 

 line only ^ an inch outwards from the points 40 and 70 ; but 

 in order to provide for paper covered, or opaque slides whose 

 uppermost and lateral edges may be used as guide-lines, the 

 graduation has been extended considerably. It will be found 

 on trial that slides of the standard size, whether paper covered 

 or not, may be employed with the Indicator for the registra- 

 tion of all objects under a glass cover of a square inch in size, 

 which is quite as large as is likely to be used. In using cover- 

 ing glasses of an inch square it will be found necessary to 

 employ the horizontal numbers to 50, and the verticals 

 A, A', for objects towards the right of the cover, and the other 

 axes and numbers for objects towards the left. For a portion 

 of the objects under the cover, either set of axes and numbers 

 may be used at pleasure, provided that the verticals are chosen 

 as far apart as possible. 



Two verticals on the same side of the centre should never 

 be used together, as a small error in observing the numbers 

 would have more effect in displacing the object from the 

 centre than if two axes at a greater distance had been em- 

 ployed. The reason for leaving a blank ungraduated space 

 between 50 and 60 on the horizontal line was to allow a fac- 

 simile of the Indicator to be engraved upon the stage of any 



