Licence: Public Domain Mark
Credit: On the modes of measuring microscopic objects / by Harting of Utrecht. Source: Wellcome Collection.
Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.
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![as from physical causes the divisions cannot be carried beyond a certain degree of minuteness, it is in vain to seek, by this procedure, such delicate measurements as are otherwise attainable. Besides, the results of this method are affected by all the faults in the construction of the micrometer; while, in most instances, and especially when high powers are used, the object and the lines (which never can be in the same precise plane) cannot be both exactly defined at the same time. The last objection is not applicable to another mode of using the glass micro- meter, consisting in adapting to opposite sides of an eye-piece two screws, ter- minating in tine points, which project into the field of vision, and can be approxi- mated to, or removed from, each other. In measuring an object, the points are screwed apart till the margins of the object correspond with their points; the object is then withdrawn, and a glass stage-micrometer substituted, in order to measure the distance between the points. This method is somewhat preferable to the former. Not only are the mar- gins of the object, the divisions of the scale, and the screw-points, seen with equal distinctness, but by bringing successive portions of the scale into the field, and taking the mean of the indications observed, the error in the workmanship of the micrometer may be made to vanish. An objection of some consequence, however, is, that time is lost in constantly shifting the object. Besides, the limits of pos- sible accuracy are as speedily reached by this as by the former method. The finest micrometric divisions on glass which the best microscopes yet made can distinguish are those of the ninth group of Nobert’s test-plate,1 in which the intervals between the lines measure millimetre = ’000026 inches. This, consequently, is the utmost limit of accuracy attainable by the glass micrometer, and it may even be assumed that the diffraction of the extreme rays will prevent this degree of exactitude from being easily reached. But the glass micrometer may be used in another way, which is in some re- spects preferable to the methods already described. Instead of being used as a stage-object, it is introduced into the ocular. A Ramsden eye-piece2 is the best, for when the micrometer is placed between the lenses of an ordinary Iluygenian or negative eye-piece, its image is too much distorted by spherical aberration. The first advantage of this method is, that in taking measures of the same delicacy by it, one may employ a glass scale more coarsely divided than when a stage micrometer is used. The reason is, that in the former case the divisions of the scale are magnified only by the eye-piece. If this magnifies ten diameters and the micrometer is divided into twentieths of a millimetre, then each division projected upon a surface at the usual distance of distinct vision (ten inches) will be equal to half a millimetre, and if the magnifying power of the whole micro- scope at the same distance be five hundred diameters, then will the value of each division on the field be yoVgtli of a millimetre. Now, as greater magnifying powers and more finely divided micrometers can be used, it is not difficult to measure in this way, directly and accurately, spaces of gyVgth to .y d^th of a millimetre = '00002 to '000016 inch : and this is greatly facilitated by the more considerable breadth and depth of the lines ruled on the eye-piece micrometer, which may be filled up with black-lead, and thus rendered more distinct. The chief advantage of the method consists in the fact, that the error in the workmanship of the scale vanishes. Thus, if it has been ascertained that a divi- sion is equal to yfrl00th of a millimetre, then if the relative difference of any two divisions is equal to one-seventh of their diameter (and this is the largest differ- ence which I have ever observed), the results of measurements cannot vary at different parts of the scale by more than -7 0l00tb of a millimetre,—a quantity which may be regarded as beyond the power of vision in most cases. 1 More recent observations with Ross’s microscopes have shown, that distances not exceeding H1^o5th of an inch can be resolved satisfactorily. See Queckett-on the Microscope, 2d edition, p. 476.—[7Vans.] 2 Two plano-convex lenses, with their plane surfaces turned outwards, and their convex surfaces opposed to each other within the eye-piece tube.—[Trans.\](https://iiif.wellcomecollection.org/image/b28043133_0007.jp2/full/800%2C/0/default.jpg)
