In " Practical Microscopy," by Dr. Shillington Scales, published in 1909, appears a very concise description of the apparatus necessary, and we extract from it some of the important points and difficulties which are associated with the work.

The resolving power of a lens depends upon its aperture, and with cedar oil as an immersion we are limited to an N.A. of about 1 -4. An immersion lens has been constructed for use with monobromide of naphthalene which has an N.A. of about. 1 -6, and proportionately greater resolving power, but this medium is, unfortunately, unsuit -able for mounting most objects, so that this lens has not come into general use. But we can increase the resolution in two other ways-either by increasing the refractive index of the mounting medium, and so shortening the wave-length of light, or by using light of shorter wave-length White light is known to be made up of rays of different wave-lengths and refrangibility, those at the violet end of the spectrum being much shorter than those at the red end. Monochromatic light, selected by means of a prism or screen, and taken from the blue, or, still better, the violet end of the spectrum, will consequently give us greater resolution than ordinary white light, which combines so many rays of longer wave-length. This is taken advan-i tape of in photo-micrography, and the result is a very definite increase in resolution-say, of the markings of a difficult diatom. Unfortunately, the eye itself is but little sensitive to such rays, so that it is not easy to see the object directly, or to focus it, though such light has high actinic value. Dr. Kohler, of Jena, has, however, experimented with ultra-violet rays of a still shorter wave-length which he finds have even greater power of resolution, but are, of course, invisible, and has lenses made of crystal and fused quartz corrected for this wave-length, which, as monochromatic light is used, have the incidental advantage of needing correction only for spherical, and not for chromatic aberration. The N.A. of the strongest system is 1 -25, which gives a resolving power, as compared with an imaginary ordinary lens used in daylight, equivalent to N.A. 2-5, on account of the small wavelength of the light used. The light itself is obtained from a 2 mm. spark between cadmium electrodes of a Leyden jar, worked with an induction machine, and light of the required wave-length is separated by an iris diaphragm, and passed through a condenser made of quartz.

" But the human eye, as has just been stated, cannot see these rays, and so cannot focus and adjust them. Therefore Dr. Kohler has devised what mav be called an artificial eye ; in other words, he constructs what corresponds to the lens of the eye-i.e., an Eyepiece, made also of quartz, and a retina or screw made of fluorescent glass, which responds to the ultraviolet rays. '1 he image on the screen is examined visually by means of an ordinary pocket magnifying ' lens ' in which case Dr. Kohler has found magnesium light to have a wave-length much more suitable than cadmium light. This lens, impervious to the injurious ultra-violet rays, lengthens them, and so renders them visible for rough examination and focusing. A photograph is then taken, showing the finer detail. It is to be noted that these quartz lenses can only be used with light of the wave-length for which they are constructed, that an immersion medium of suitable refractive index (made of glycerine and water in proportions calculated to give such a definite refractive index) must be used, and that the cover-glasses and slides must also be made of fused quartz, or the latter can be made of ultra-violet permeable glass."

For comparative purposes it must be stated that Objectives composed of quartz lenses having numerical apertures respectively of -35 and -85 would give, if illuminated by ultra-violet light a s already described, relative apertures of -70 and 1 -7 respectively.

It has to be remembered that this work is limited to photography only, that it requires a number of special lenses, mountings and fittings generally for the exclusive purpose, and consideration would have to be given to any value which might attach to its use before a purchase were embarked upon.

It must not be overlooked that Mr. Barnard says in his contribution to the " Lancet " on the subject that " Focussing by means of the fluorescent image is both difficult and dangerous owing to the action of ultra-violet light on living organisms. Changes can be set up which might be regarded as normal appearances in the organisms. The actual exposure, therefore, of any material to ultraviolet must be the shortest possible. There is no reason to regard the action on living organisms as a trigger action, a certain time factor is involved, but precautions must be observed to ensure that no change due to the light itself is set up."

Dark Ground Illumination

Of much greater practical value is the dark ground illuminator, and especially that pattern which can be used with any Oil Immersion Objective utilising its full aperture without a funnel stop in the Objective to reduce its efficiency, and which is effective through the thickness of an ordinary 3-inch by 1-inch slip. Such is the Nelson Cassegrain of Watson's make. Mr. Barnard remarks : " In the light of recent experiments it is, in fact, clear that full resolving power is obtained more certainly by an efficient dark-ground illumination method than by any other means," and this undoubtedly is true. The effects obtained by its means are so widely and appreciatively known that full details are not necessary. It is now made in a new form of quartz for ultra-violet light.