Page:Popular Science Monthly Volume 61.djvu/152

146 the object in general obtains when the Abbe condenser is as far away from the microscopic slide on one side as is the objective on the other. Thus for oil immersion work a drop of oil must be placed between the condenser and slide as well as upon the objective. It frequently happens that when the proper position of the condenser is located for low power objectives the detail is somewhat obliterated because of too intense illumination, and the manipulation of the iris diaphragm connected with the condenser is necessary in order to obtain the best results.

From whatever source the light falls upon the Abbe condenser, an auxiliary condenser before it is advantageous, except when daylight is used upon the mirror beneath the stage. The use of daylight has disadvantages, however, which do not recommend it for photographic work. With diffused light falling upon the mirror it is impossible to use even moderately high power objectives and obtain sufficient light for focusing,

while with the direct rays of the sun, unless a heliostat is used to maintain the position of the sun constant upon the mirror, one cannot obtain a photograph before the sun has changed enough to throw the image of the object off the plate. In addition to this inconvenience in general, the optical imperfections of the mirror are such as to make the production of sharp photographs difficult, and much better results are obtained with high magnifying power, by dispensing with its use and transmitting the light directly from the source through the optical system.

Photomicrographic work of low magnification has been successfully done with the various forms of artificial illuminants of low candle power. The writer, in the production of first photographs used very