On a universal indicator for microscopes.

  • Bailey, Jacob Whitman, 1811-1857.
Date:
[1855]
Catalogue details

Licence: Public Domain Mark

Credit: On a universal indicator for microscopes. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

    6/14 (page 4)
    It was desired to make the instrument capable of universal ajv plication; so simple that it could be adapted to any stage ; so light and yet so strong that it conid be sent without injury by mail or otherwise to any distance; and lastly that the ditTerenl copies should be perfect fac-similes of each other and reprodncible at any lime. All this is secured by having the Indicator engraved upon a steel plate and printed upon cards of uniform qnality, and by taking the dimensions from the standard United Sjates 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 ar- ranged 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 ne- cessary for their use would only extend upon the verticals ^ an inch above and below the horizontal line, and upon the horizon- tal line only h, an inch outwards from the points 40 and 70; but in order to provide for paper covered, or opaque slides whose up- permost and lateral edges n}ay be used as guide lines, the gradua- tion has been extended considerably. It will be found on trial that slides of the standard size, whether paper covered or not, may be eiuployed with tlie Indicator for the registration 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 covering glasses of an ii]ch square it will be foimd necessary to employ the horizotital numbers 0 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 letl. 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 employed. The reason for leaving a blank ungraduated space between 50 and 60 on the ho- rizontal line was to allow a fac-simile of the Indicator to be en- graved upon the stage of any microscope, the blank space being left for the portion of the stage occupied by the aperture. The guide lines upon the glass may be ruled with a fine pointed scratching diamond, and be rendered more visible by having oraphite or black lead rubbed into them. Lines ruled in this man- ner will answer for all except very miniUe objects, but in conse- quence of the widening of the lines by the chipping up of the glass due to unequal expansions and contractions, the lines often be- come too wide and irregular for very accurate adjustments. In such cases admirable guide lines may be etched upon the glass