76 THE NORTHERN MICROSCOPIST. 
DESIGNATION. DIAMETERS. 
Without With the A 
Eye-piece. Eye-piece. At Io ins. 
A Teh eh I2 36 18 
Oe Be Ao es 21 63 32 
Teas. 37 I1O 56 
eine 80 240 120 
ra Blige ee 173 520 260 
meee saris, €. 360 1000 540 
RS RETA 530 1600 800 
15 
The microscope employed may be of any ordinary kind, 
monocular preferably, and if provided with a means of shortening 
the tube or body so much the better. The tube should be lined 
with velvet, to prevent central flare, and the instrument provided 
with a coarse and fine adjustment, the latter answering the helm 
when turned either backwards or forwards; but more particularly 
must it be sensitive to very slight movements of the milled head 
when withdrawing the objective from the object. 
Dr. Woodward, in his report already quoted, writes, “the 
objective selected should always be specially corrected for photo- 
graphy.” I cannot agree with Dr. Woodward on this point; no 
doubt it is very convenient to have one so corrected, but it is by 
no means imperative. To those as yet uninitiated in the mysteries 
of the photographic art, it may be necessary to say that the vzswal 
and actinic foci often (though not necessarily) lie in different planes, 
_ and therefore an objective which gives perfect definition 40 che eye, 
will often produce a blurred and indistinct image upon the sensi- 
tive plate. This may be remedied by withdrawing the objective, 
by turning the milled head of the fine adjustment ; it cannot be 
told beforehand what allowance is necessary, the only method is 
by trial and error, some objectives require no correction whatever, 
while others which necessitate considerable movement from the 
object can be made to yield just as perfect results. 
The best way to arrive at the necessary correction is to take a 
picture at the best viswal focus, develop, fix and dry it in the usual 
manner, and then withdraw the objective from the stage by means 
of the fine adjustment until the image appears on the ground glass 
about as indistinct as it does in the negative. Ordinary ground 
glass is too coarse to focus upon, two surfaces should be rubbed 
together with a little of the finest emery and water, or a focussing 
glass may be prepared by coating an ordinary glass plate with spirit 
varnish, and allowing to dry without heat, or a coating of unboiled 
rice starch may be applied and allowed to dry on a levelling 
stand. The correct focus may be obtained by the eye alone, 
nevertheless, it is always advisable to use a magnifier, an engraver’s 
lens, or an ordinary eye-piece with the eye-lens removed. 
