PHOTO-MICROGRAPHIC APPARATUS. 41 
Ramsden or Aplanat (Zeiss) type. The front of the 
plain glass should have lines marked on it with a 
diamond, and it will be found very useful to have these 
lines made at intervals of one-tenth of an inch, or such 
equivalent in centimetres as may be suitable. By 
means of these lines, the size of even very minute 
objects may be measured, almost at a glance, if the 
magnification at which we are working is known. The 
Ramsden, or Aplanat, must, of course, be focussed once 
for all on the lines on the glass; if it is found difficult 
to do this, some such object as the wing of a fly may 
be fixed pro tempore to the front. Our friend, Dr. E. C. 
Bousfield, has recommended to us, and we use with 
great advantage, for focussing at higher magnifications, 
a double convex spectacle-lens of about 8in. focal 
length. This we have mounted in a rigid tube of such 
length that the lines are at its focal point, and we lay 
this on the plain glass and put our eye at such a point 
that the field of the lens is seen full of light; by this 
procedure we obtain a most accurate focus for high 
power work ; but for low magnification the light is too 
strong for our eye, lime-light being used. While this 
is a description of our own instrument, it is to be 
understood that, if the reader keeps in mind the points 
we have laid down as absolutely essential to a good 
apparatus, he will, doubtless, find on the market other 
instruments worthy of his confidence. Thus Messrs. 
Swift make apparatus less costly than that figured, 
and Messrs. Beck, Crouch, and Watson, in London, 
D 
