62 Transactions of the en ee ee 
observed, whilst at the same time a horizontal stage is preserved, 
notwithstanding the inclination of the body of the instrument at 
any angle which by experiment may be found the most convenient 
in use—the object of the latter arrangement being to enable the 
observer to dissect, or arrange, objects under fluids, a process which, 
with an inclined stage, is obviously impossible. 
The former object is attained in the following manner. 
The light emerging from the objective is received on the shorter 
sides of two truncated rectangular prisms (Fig. 1), of which the sides 
enclosing the right angles are equal ; the hypothenuse planes of these 
prisms being placed together, but inclined to each other at an angle 
of about 4 degrees, form a wedge of glass, having an angle of about 
94 degrees, and on this wedge the diverging cone of light falls. The 
light incident on the first surfaces (B C) of the prisms is thus divided, 
and being refracted towards the longer sides A B is thence totally 
reflected and emerges at the upper sides A D; the dispersion caused 
by refraction at the first, being corrected at the second transmitting 
surface. 
We have thus obtained two fields, each having, by one reflexion, 
been laterally inverted. 
In order to perfect the erection of the image it is now only ne- 
cessary to induce a second reflexion in a plane at right angles to 
the first, giving what I have called, for want of a better expression, 
the longitudinal inversion ; this is readily accomplished by placing a 
third prism (Fig. 2) over those just described, and may be constructed 
to reflect the light at any angle which, as I said before, may be deemed 
the most convenient in use, although it is obvious that the nearer 
this approaches a right angle, the smaller will be the quantity of 
glass employed, and the less the consequent loss of light. It is 
sufficiently obvious from the figure .that the two laterally corrected 
pencils of light entering the prism at BC are totally reflected at 
A B, and emerge longitudinally inverted at A C. 
In the instrument before you this angle is 75 degrees (the others 
being of course 52} each), which is probably, with this stand, as 
convenient as any. 
By the combination thus complete, the two perfectly-erected 
fields reach the eyes of the observer at an angle of 15 degrees 
(being the complement of that previously indicated), which enables 
him, with his arms resting on the table, to operate with fluids on a 
still horizontal stage, the distance of which, from the eye-piece, has 
been lessened rather more than an inch by the bent course which the 
light has been compelled to travel. At the same time the instru- 
ment, being perfectly symmetrical, the illumination of each field is 
exactly equal, and the focal distance the same. 
Having thus imperfectly described the mode by which the two 
essential conditions have been fulfilled, I will now direct your at- 
ee eee 
