Illuminating Apparatus and Methods of Illumination

Many object-glasses are unsuitable for use with a vertical. illuminator owing to "vertical illuminator glare," i.e., the produc tion of "flare images" which are brought into focus, or nearly into focus, by the eyepiece. These images, which are due to reflections at the various lens surfaces, are superposed on the image of the object, and may render it impossible to see this image properly. Object-glasses for use with a vertical illuminator should be se lected for their freedom from this flare effect.

The Ring Illuminator.

An illuminator, which provides "top" illumination without using the object-glass as part of the illuminating system, has been introduced by the firm of R. and J. Beck. This illuminator is an annular lens-mirror reflector. In principle this "ring" illuminator acts in the same way as the While the optical system by which the image is obtained is the most important part of a microscope, the fullest use of this system cannot be made unless the microscope stand is accurately constructed and arranged for the convenient manipulation of the optical parts, the object, and the illuminating system.

The Body Tube and Its Mounting.

The object-glass and the eyepiece are mounted in a body tube, the length of which can be varied so as to allow of the tube-length being adjtsted to suit the corrections of the object-glass. The object-glasses are screwed into the bottom end of the body tube and are interchangeable one with another; the eyepieces are made to slide into the upper end of the tube. The body tube is mounted on slides carried by the main limb of the microscope frame, movement of the tube rela tive to the limb being provided by the coarse and fine adjustments. The coarse adjustment is operated by a rack and pinion mechan ism which moves the coarse adjustment slide up and down in machined guides. This type of design is almost universally em ployed. The fine adjustment slide also works in machined guides, but is operated by a mechanism which is capable of adjusting the position of the body tube by very small amounts. The design of the fine adjustment has attracted the attention of microscope makers for very many years, and the num ber of designs invented is so great that to describe them here is out of the question.

In some instruments the slide of the fine adjustment block fits in guides in the limb, the coarse adjustment slide being car ried by guides machined in front of the fine adjustment block. In other instru

ments this arrangement is reversed. At one time designs were developed in which only the object-glass fitting was carried by the fine adjustment, the object-glass fitting being made to move up or down relatively to the body tube. These designs have practically disappeared.

The fine adjustment should move easily and smoothly, be free from "backlash" and from tendency to stick, and should hold the microscope steady. In microscopes of good make, the body tube can be set by means of the fine adjustment to within 26,1000 in. or, in some cases, to in., and can be brought back to its original setting to within similar limits if the fine adjustment is re-set to its original reading after having been altered. In a special instrument made by one English firm, for ultra-violet work, the fine adjustment can be set by means of its indicator so as to bring the body tube to within PR--)!Tir. in. of any desired position.

Binocular bodies to allow of the simultaneous use of both eyes are commonly met with. Some instruments are provided with binocular bodies which are interchangeable with the ordinary single-tube (monocular) body. In the binocular bodies prismatic systems are arranged close behind the object-glass, so that light from the object-glass is brought up as two beams of approxi mately equal intensity, one beam into each eyepiece. Matched pairs of eyepieces are used.

If both eyes receive light from the whole of the object-glass aperture, stereoscopic relief is not obtained. To obtain stereo scopic relief, the light coming from the right half of the object-glass must be brought up to the left eye, and vice versa, unless the prism system completely inverts both of the ray systems from the object glass so as to produce erect images of the object. The binocular prism invented by Wenham in 1861 is the simplest yet de vised (1928) for easily giving proper stereoscopic appearances, and this form of prism is commonly used in modern binocu lar instruments. Wenham's prism brings the rays from the right side of the object glass up to the left eye, the rays from the t left side being allowed to pass on unde e viated so as to enter the right eye (fig. 28). The images formed are inverted images in the object, and show proper stereoscopic relief. Many of the early systems did not invert the ray systems so as to produce erect images of the object and gave rise to pseudoscopic effects.