MEMORANDA. Vat 
before it. The field of view will be the base of an angle, 
which appears equal to the angular aperture. To measure 
this angle, place the central point of the straight edge a gra- 
duated semicircle, exactly under the edge of the lowest com- 
bination of the objective, the convexity being towards the rule 
or wall. The number of degrees included between two lines 
drawn from that point to the extreme limits of the field of 
view, will be the measure of this angle. 
The following table is, 1st, by this method ; 2nd, after Mr. 
Pritchard, as described by Dr. Lardner ; and, 8rd, as given 
by the maker. The adjustment ring ina line. I think that 
by the second method you by no means get the full angle with 
the two and one-inch. 
Table of Angular Aperture of Objectives. 
Ist. 2nd. ord. 
4 128° 126° 125° 
} 108° 105° 106° 
oD: 58° aye 60° 
il 22° 16° Do 
oy RG? +O° 15° 
Ricnarp Nicuots, M.R.C.S. 
Sr. Heipy’s, JERSEY. 
Micro-Stereographs,—In ‘ Quart. Journal of Micro. Science,’ 
No. 3, for April, 1853, Professor Riddell gives the first an- 
nouncement of a binocular microscope, and mentions its ap- 
plicability for obtaining “ match drawings” with the camera 
lucida, to be viewed with the stereoscope; and again, in 
No. 5, p. 24, for the same year, he says, “ If the same object 
be drawn as seen, through each ocular respectively, a differ- 
ence between the two drawings is perceptible, similar to that 
between match stereoscopic pictures; so that if these two 
drawings be viewed each with the appropriate eye, the natural 
relief of the object is reproduced.” 
In ‘Trans.’ of the Micro. Society (‘Quart. Journal of 
Micro. Science’), No. 10, Jan., 1855, pp. 5, 6, in a paper on 
“Microscopic Photography,” I described a method of obtain- 
ing micro-stereograms without shifting the object, simply by 
obscuring the alternate halves of the object-glass, by means 
of a sliding stop. I had before this abandoned the use of the 
camera, and given the preference to the image obtained 
through the shutter of a dark room, by means of a Heliostat ; 
in fact, using the ordinary microscope as a solar one. ‘This, 
besides other reasons, gives the very important one of allow- 
ing the light to be adjusted while the sensitive plate is in 
