118 THE MICROSCOPICAL NEWS. 
each variation of the bellows. Four results will thus be exhibited 
on the same plate. ; 
No tables can be given for the actinic allowance required by the 
various powers. A one-inch by Dancer, lent by a friend, was found 
to produce sharp pictures without any such allowance, whilst an- 
other by Swift required 2° on my scale. Powers above one-quarter 
of an inch seldom require compensation, and some of the phofo- 
graphic \enses of English make, when stopped down, perform 
admirably on large objects of a half or one inch in diameter. 
Many of the pictures to be seen to-night were produced by Dall- 
meyer’s stereoscopic lens, kindly lent by Mr. J. Pollitt. The tyro 
is recommended to ascertain - definitely, once for all, the exact 
allowance required for each lens, and to keep a careful record of 
the same. 
In order to produce satisfactory negatives no pains should be 
spared to obtain a flat and even picture, by levelling the object by 
means of the set screws referred to. I have seen many otherwise 
commendable photographs utterly spoiled by one-half of the picture 
being out of focus. 
Lllumination.—The most preferable source of light, as far as my 
experience goes, is the sun, but the electric arc, lime-light, gas, and 
paraffine lamps have all been used. Now that sensitive dry plates 
are within the reach of all the paraffine lamp is usually employed, 
either naked or with a bull’s-eye condenser interposed. If the 
latter be employed difficulty is often experienced in obtaining even 
illumination all over the field, the thickness of the glass breaking 
up the rays into prismatic colours. Mr. Dancer recommends a 
double combination quarter-plate lens used as a condenser, an 
image of the lamp flame being formed thereby a little behind the 
object, so that the rays just cross before arrival. <A disc of light 
should be produced which amply covers the size of the object. It 
will be found convenient to ascertain the correctness of the illumi- 
nation by placing a slip of white writing paper on the object-slide, 
and observing whether the image of the flame fully envelopes the 
field. In using the higher powers, when it becomes necessary to 
obtain more intense light upon a small space the paraffine flame may 
be placed edgewise. With a good lamp powers up to one-fifteenth of 
an inch may be employed with dry plates. For low magnifications 
ordinary daylight from a white cloud will suffice. In using sun- 
light it is sometimes necessary to interpose a glass cell containing 
a solution of common alum to arrest the heat rays, otherwise the 
object or the lens may be injured. With naked sunlight, also, 
diffraction and interference lines are apt to appear around the 
image, when a plate of ground glass should be fixed an inch or so 
behind the object to soften the light. Dr. Woodward, in using very 
high powers, has sometimes found it imperative to make use of 
