The Beck Ultra-violet Microscope made for J. E. Barnard, Esq., F.R.S.

Many years ago it was realised that if ultra-violet light could be used in connection with the microscope it would be possible to at least double the resolving power of the instrument. An attempt was made to solve the problem, but it was practically abandoned owing to the difficulties of manipulation. It was not until Mr. Barnard worked out a completely new technique that the principle was brought into practical use so that photographs could be taken with almost the same certainty and regularity as with ordinary light. Special apparatus was required of a totally different type from that originally suggested, and the following is a description of the apparatus which we manufactured to Mr. Barnard’s instructions for this work.

There are but few substances that are transparent to ultra-violet light of a reasonably short wavelength. At present there is no such glass, and most of the natural materials which allow ultra-violet light to pass are crystalline. The only substance which is available at present for manufacturing lenses for this apparatus is fused quartz. An achromatic lens cannot be made composed of only one material. The lens must therefore be monochromatic and will only give a clear image when light of one wavelength is used. The other aberrations can be corrected by means of a number of lenses used in combination, the shapes of which are specially calculated, but it can only be corrected for one specified wavelength of light.

The monochromatic lens made by us has been designed and computed by the Biitish Scientific Instrument Research Association. It is corrected for ultra-violet light of 2,753 A.U. The spectrum of cadmium contains a number of lines in the ultia-violet of different wavelengths. One of these has the above wavelength for which the object glass is corrected. This must be isolated by means of a train of prisms or some other suitable apparatus. It will work well for light slightly different from 2,753 AU if a suitable change in the tube length is made.

The focus of a monochromatic lens is in a different position for every wavelength of light. If it were possible to obtain a clear image when using light of some particular wavelength in the visible spectrum it would be possible to alter the focus of the microscope by a specified amount to obtain the correct position for the ultra-violet light for which it is corrected. The image produced by any visible light is, however, not sufficiently clear to enable an accurate setting of the focus to be made. This difficulty has hitherto formed one of the great obstacles to getting satisfactory photographs.

A fluorescent eyepiece has been used whereby the invisible ultra-violet light was turned into visible light by means of a fluorescent screen. Experience has shown that the image on a fluorescent screen is not sufficiently clear to make it possible to set the focus with any precision.

The technique devised by Mr. Barnard consists of having a quartz monochromatic lens, for use with the ultra-violet light, and an apochromatic lens, for use with visual light, of the same focal lengths. Both are carried in mounts so adjusted that the distance from the object to the back of the mount when using ultra-violet light is almost the same as that with the apochromatic when using visual light.

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