When any of these eye-pieces are applied to a telescope, with the lens E on the upper part of it, we look down upon the object, if it be a terrestrial one, as if it were under our feet. If we turn the eye-piece round in its socket a quarter of a circle towards the left, an object directly before us in the south, will appear as if it were in the _west_ and turned upside down. If, from this position, it is turned round a semicircle towards the right, and the eye applied, the same object will appear as if it were situated in the east, and inverted; and if it be turned round another quadrant, till it be directly opposite to its first position, and the eye applied from below, the object or landscape will appear as if suspended in the atmosphere above us. This eye-piece, therefore, is capable of exhibiting objects in a great variety of aspects, and the use of it is both pleasant and easy for the observer. But there is a considerable loss of light, occasioned by the reflection from the speculum, which is sensibly felt when very high powers are applied; and therefore when very small stars are to be observed, such as some of those connected with double or triple stars, the observer should not study his own ease so much as the quantity of light he can retain with a high power, which object is best attained with an ordinary eye-piece and a telescope of large aperture.
We have said that a diagonal eye-piece may be constructed with a reflector _before_ the eye-piece. In this case, the speculum is sometimes made to slide before the eye at the requisite angle of reclination, in which application each eye-piece must necessarily have a groove to receive it, and the eye must be applied without a hole to direct it, but it may be put on and taken off without disturbing the adjustment for distinct vision, and is very simple in its application. But, on the whole, the form represented in fig. 78, is the most convenient, and should generally be preferred, as any common astronomical eye-piece can be applied to it. I have used a diagonal eye-piece of this kind, with good effect, when a power of 180 has been applied to the sun and other celestial objects.
Instead of a metallic speculum, a _rectangular prism of glass_ is sometimes substituted; for the rays of light are then bent by reflection from the second polished surface, which ought to be _dry_, and undergo two refractions which achromatise them; and the same effect is thus produced as by polished metal. Ramsden sometimes gave one of the polished faces of a right angled prism a curve, which prism served instead of a lens in an eye-piece, and also performed the office of a reflector. A semi-globe, or what has been called a Bull’s eye, has also been used as a diagonal eye-piece, and when the curve is well-formed, and the glass good, it is achromatic, and is said to perform pretty well, but it is not superior to the forms already described.