After having focussed with visual light with the apochromatic lens and having changed to the 
monochromatic lens in order to use ultra-violet light, only a small adjustment has to be made, which 
is in the nature of one-fiftieth of what would otherwise be the case if the lenses were not so mounted. 
For carrying this out extraordinary rigidity in the whole of the apparatus is necessary combined 
with a means of interchanging the lenses with great certainty. A fine adjustment of far greater 
delicacy than has previously been supplied on a microscope is required. It may safely be said 
that even the best microscopes hitherto made are quite unsuitable owing to lack of rigidity and 
uncertainty in the focussing adjustments. With this special microscope a fine adjustment is 
provided in which there is no perceptible backlash, and in which every division on the milled head 
moves the object glass 1/10,000 of a millimetre. 
Searching a specimen for the portions to be photographed or finding any particular organism 
by means of this invisible light obviously cannot be done by photography. The difficulty is 
overcome by making the monochromatic and apochromatic lenses so exactly concentric and so 
certain in their interchangeable mounts, that each has the same field of view. The condenser 
with which the object is illuminated is duplex. The central portion is of quartz transmitting 
ultra-violet light, the outer portion is a glass dark ground illuminator transmitting visual light. 
The dark ground illuminator has an independent focussing adjustment so that the visual and the 
ultra-violet light may be set to the same focus and the whole condenser then focussed by means of the 
substage focussing adjustment. The visual light from the dark ground illuminator and the 
apochromatic lens is used for the visual examination of the specimen for finding the objects to be 
photographed, for placing them in the correct position and for determining their general 
characteristics, after which all that has to be done in order to take the photograph is to change the 
object glass, alter the slow motion by a small pre-determined number of divisions, attach the 
camera and take the photograph. 
If it is considered to be undesirable to use the ultra-violet illumination during the process of 
preliminary examination in order that it should not kill the organisms being examined, the ultra- 
violet light may be turned on only for the actual purpose of making the exposure. The fluorescent 
eyepiece although it is not used for focussing is of use for the arrangement of the illumination. 
The instrument is so made that this duplex condenser can be changed for a dark ground 
illuminator by means of a dovetailed slide in the substage into which both pieces of apparatus 
drop interchangeably. This ultra-violet dark ground illuminator is a reflecting apparatus made 
from a metal which has a specially high reflecting power for short wavelength light. Silver and 
many metallic reflectors have so little reflecting power for these radiations that they are useless. 
The camera is made to attach to the bedplate which carries the whole apparatus. It is not 
connected with any portion of the microscope, and consequently can be fitted on without any fear 
of interfering with the adjustments. 
There are certain other factors in the use of ultra-violet light that do not arise in ordinary 
photography. For instance, if the object, the mounting material, the slip, the cover slip or any of 
the optical elements are fluorescent, they may render the object invisible. In one case a very 
small percentage of carbolic acid placed in the mounting medium for preservative purposes was 
rendered fluorescent by the ultra-violet light and caused the background to be so bright that the 
object was entirely obscured. There must be no grease present on any of the surfaces as most 
forms of grease are fluorescent. The immersion fluid of the immersion lenses cannot be the usual 
cedar wood oil, and no cement at present known can be used on any of the lenses or slides as they 
are opaque to the light used. 
The immersion fluid used is a definite proportion of glycerine and distilled water, and precautions 
must be taken to ensure that the evaporation of the water or the hygroscopic quality of the 
glycerine do not alter the refractive index of the immersion fluid, which must remain exactly the 
same as that of fused quartz for the 2,753 A.U. line. If dark ground illumination is employed, 
the immersion fluid must be filtered to remove fine particles which otherwise by scattering light 
destroy the blackness of the background. 
For dark ground illumination the exact centration of the illuminator is of the utmost 
importance, and the final adjustment should be made by observing the concentricity of the diffraction 
rings caused by a brilliant illumination on a small piece of dust in the object, or better still of a 
minute mercury globule. 
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