Fig. 120.—Section of Prism Binocular.
The path of the rays is plainly shown and the manner in which the prisms fold up the total focal length of the objective is quite obvious. The added stereoscopic effect obtained by the arrangement of the two sides of the instrument is practically a very material gain. It gives admirable modelling of the visible field, a perception of distance which is at least very noticeable and a certain power of penetration, as through a mass of underbrush, which results from the objectives to a certain extent seeing around small objects so that one or the other of them gives an image of something beyond. For near objects there is some exaggeration of stereoscopic effect but on the whole for terrestrial use the net gain is decidedly in evidence.
A well made prism binocular is an extremely useful instrument for observation of the heavens, provided the objectives are of fair size, and the prisms big enough to receive the whole beam from the objective, and well executed enough to give a thoroughly good image with a flat field.
The weak points of the prism glass are great loss of light through reflection at the usual 10 air-glass surfaces and the general presence of annoying ghosts of bright objects in the field. Most such binoculars have Kellner eyepieces which are peculiarly bad, as we have seen, with respect to reflected images, and present the plane surface of the last prism to the plane front of the field lens. Recently some constructors have utilized the orthoscopic eyepiece, Figure 105a, as a substitute with great advantage in the matter of reflections.
The loss of light in the prism glass is really a serious matter, between reflection at the surfaces and absorption in the thick masses of glass necessary in the prisms. If of any size the transmitted light is not much over one-half of that received, very seldom above 60%. If the instrument is properly designed the apparent field is in the neighborhood of 45°, substantially flat and fairly evenly illuminated. Warning should here be given however that many binoculars are on the market in which the field is far from flat and equally far from being uniform.
In many instances the prisms are too small to take the whole bundle of rays from the objective back to the image plane without cutting down the marginal light considerably. Even when the field is apparently quite flat this fault of uneven illumination may exist, and in a glass for astronomical uses it is highly objectionable.
Before picking out a binocular for a study of the sky make very careful trial of the field with respect to flatness and clean definition of objects up to the very edge. Then judge as accurately as you may of the uniformity of illumination, if possible by observation on two stars about the radius of the field apart. It should be possible to observe them in any part of the field without detectable change in their apparent brilliancy.
If the objectives are easily removable unscrew one of them to obtain a clear idea as to the actual size of the prisms.[19] Look out, too, for ghosts of bright stars.