228 MEMORANDA. 
is procured. ‘The second mode, of making the upper row of 
figures represent longitude by aid of a line of separation, is 
liable to great objection from its being easily omitted acci- 
dentally when intended to be drawn. On the whole, then, 
the third mode (any of the variations), which is sufficiently 
accurate for enabling any one to discover the object, appears 
to be the simplest; but whichever mode is to be adopted, 
whether for mere notation or for the position of the slide, 
uniformity is necessary, and it ought to go forth to the public 
with the authority of the Microscopical Society of London, or, 
at least, of Mr. Maltwood himself. From want of some 
definite method J have had sometimes to spend two or three 
hours in search of a registered object sent me before I dis- 
covered it or the system of notation employed, whereas an 
ink-dot would have brought me to it in a few seconds. 
T need scarcely add that, in registering or finding objects in 
a slide, direct light from the mirror ought only to be used, 
the effect of oblique hight, or of a condenser of any kind, 
being to alter the apparent position of the object, sometimes 
to a very considerable degree if a low power be not used. 
es 
March 20, 1865. 
Beck’s Treatise on the Microscope.—I have lately been read- 
ing with some attention-and much interest Mr. Richard Beck’s 
‘treatise on his achromatic microscopes; for though at first 
sight it may appear a mere illustrated trade catalogue, a more 
careful inspection will convince any one that it contains many 
valuable and practical hints relative to illumination and ma- 
nipulation ; whilst, besides the plates exemplifying his own 
instruments, there are more than a dozen others representing 
objects of general interest. These are carefully and beauti- 
fully executed, especially those of the Arachnoidiscus and the 
section of deal. But I do not refer to his book for the pur- 
pose of praising it, but with the view of criticising two state- 
ments contained in it. 
At p. 30, in speaking of the appearance of the Podura scale, 
as seen by reflected light under the 1-8th and 3rd eye-piece, 
Mr. Beck says—“ We have the following results with this 
particular object, as shown in pl. xi, fig. 2, the arrow at the 
left-hand side indicating the direction of the light. When 
the markings are at right angles to the direction of the light 
(as at A), they are illuminated on the side furthest off; when 
they he in the same direction as the light, with their narrow 
