60 
POPULAR SCIENCE REVIEW. 
of having too much light, barely enough can be obtained, and 
no means of concentrating it are to be neglected. 
I will suppose magnesium to be the light employed ; the point 
at which the wire is to burn must be carefully adjusted so that 
the ignited portion, about a quarter of an inch in length, may 
be exactly opposite the pinhole stop (P), and give a clear and 
even disc of light on the focussing screen at any length of 
camera. The next point is the arrangement of the condensers, 
and on this head little can be said to lighten the trouble of ex- 
periment. Probably every object will require some variation in 
the position of the lenses, and the best guide is the appearance 
of the image on the focussing screen. It should be evenly illu- 
minated, clear, sharp, and distinct in outline. With the lower 
powers, probably the highest light attainable short of focussing 
the flame on the object will be found to answer best; but in 
representing the finer markings of Diatomacese, the light should 
in general be sacrificed to definition, and this may be done either 
by placing condensers and diaphragms in such a position as to 
exclude all oblique rays, which give an illusive brilliancy to the 
image on the screen, or by cutting off the parallel rays by a cen- 
tral stop, in the one case using the parallel rays of the central 
pencil only, in the other securing the more marked definition 
given by oblique rays, which may be thrown in any desired direc- 
tion by patterns cut out of a pill-box cover, and fixed on to the 
condenser.* 
Before proceeding further, it will be found convenient to 
settle the lengths of camera which correspond to different 
degrees of enlargement. 
This may be done by placing a stage micrometer as an ob- 
ject, and focussing it by means of the oil-lamp mounted in place 
of the magnesium wire. The camera is then lengthened or 
shortened, until the lines on the screen are found enlarged to 
the required degree. 50, 100, 200, and 400 diameters are use- 
ful enlargements, and the length of the camera for these, with 
the several lenses, should be carefully noted. 
“ Turning outT — When these preliminaries have been con- 
cluded, it will be necessary to prepare the photographic mate- 
rials, and ascertain experimentally the amount of correction 
required for each separate lens. Much time will be occupied in 
this, and the operator’s patience will be tried to the uttermost; 
but no care can be too great, no attention too exact, at this 
stage of the process — an error here may cause a series of dis- 
appointments, extending over weeks or months of w r ork. There 
can be no certainty in the results unless the “ turning out” be 
ascertained at once and without a shadow of doubt, and faulty 
Cf. Journal Microscop. Soc. vol. vi. p. 10, 
