^jo^lTjanSTS?'] The Binocular Microscojpe. 81 
obtain the adjustment for decHnation. Such an arc is represented 
in the drawing, and if used, should be graduated as follows : — The 
indicator being a line drawn on the edge of a fenestrated opening 
in the arm e, the zero line on the arc should be taken at 45° from 
the diameter of the arc which passes through the plane of the 
mirror's surface. Then graduate the arc into half-degrees for 24 
half-degrees to each side of the zero line ; number each half-degree 
1, 2, &c., counting each way from the zero line. Then each half- 
degree to the left or right of the zero line gives the necessary 
adjustment of the mirror for each whole degree of the sun's declin- 
ation north or south respectively. To make the adjustment there- 
fore, turn the mirror until the indicator points to the number of 
the graduations on the arc which represents the actual degree 
of declination of the sun for the given day. 
In the drawing the heliostat is represented standing upon a 
stool on the outside window-ledge of a window having a south 
exposure, and the dotted lines show the direction of the sun's rays 
reflected from the heliostat mirror upon a secondary mirror, and 
thence horizontally into the room. This secondary mirror may be 
conveniently mounted on a rod fixed in the window-shutter. 
V. — On some Modifications of the Binocular Microscope. By 
M. Nachet, of Paris. 
(Communicated by Henky Lee, F.L.S.) 
At the meeting of the Eoyal Microscopical Society on the 13th of 
May last, Mr. Heisch objected to my binocular arrangement : — 1st, 
that it had the effect of modifying the focal dis- 
tance of the movable body (or tube) ; 2ndly, that 
the fittings for carrying the prism and the body 
are liable to become loose by wear. Since last 
year I have remedied both these defects by a 
new arrangement, which is represented in the 
accompanying drawing. By this plan the angle 
of the tubes changes by the action of the screw 
V, and consequently the distance of the eye- 
pieces is very easily adjusted. The screw V has 
two threads of different speeds, so that the in- 
clination given to the prism itself is half the 
displacement of the tube. This arrangement is 
necessitated by the fact that the displacement 
of the rays reflected by a rotating surface is 
double the angle described by this surface. The 
solidity of this apparatus is absolute, and the 
proper distance for the eyes may be obtained without altering the 
