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Page:Popular Science Monthly Volume 61.djvu/151

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in order to obtain the desired result. Of course with low power objectives one may do this after repeated trials with some accuracy, but the chief difficulty is found in determining whether an indistinct negative is due to improper manipulation, or imperfect resolving power in the objective. For this reason, even for elementary work, such apparatus is not economical and is quite unsatisfactory.

A consideration of the apochromatic objectives of Professor Abbe and Professor Hastings, which are readily obtained with either the Zeiss or Bausch & Lomb Optical Co.'s instruments, is therefore important. These objectives having a uniform correction for spherical aberration, correct also for three colors, resulting in a better concentration of image rays and greater resolving power; this improvement in objectives brings the photographer's work nearer perfection, though there is, with high power objectives, an error which thus far remains uncorrected. This defect is balanced, however, by over-corrected eyepieces known as compensating oculars, which are designed for use with these objectives. Thus for photographic work neither an apochromatic objective with the ordinary Huyghenian eyepiece, nor the achromatic objective with compensating eyepiece is satisfactory.

The bulk of literature on photomicrography advises the use of the objective without any eyepiece; such advice is good with ordinary achromatic objectives, as the addition of the eyepiece would only introduce more absorbing and reflecting parts without correcting any of the defects in the objective. Where fine work is desired, however, and apochromats are used, the compensating ocular is not only necessary for the highest degree of correction of the system, but is useful in the regulation of magnifying power.

From the fact that so many different makes of objectives and oculars are in the market, accompanying their respective microscopes, the simplest method of rating in photographic work seems to be a statement of the number of diameters which the object is magnified. The magnifying power is most readily and accurately obtained by using a stage micrometer, one millimeter divided into hundredths, and measuring directly the magnification of the image upon the ground glass with an engine-divided steel rule.

The illumination of the object to be photographed is of so much importance that a few points may be briefly considered. An almost indispensable adjunct of the microscope in this connection is the Abbe sub-stage condenser, which not only condenses the light, but illuminates the object with a cone of light having an angular aperture equal to that of the objective employed. It is arranged to move with rack and pinion, thus providing a means for controlling the illumination to an important extent, Experience shows that the best illumination upon