No. 3293. The Beck patent focussing dark ground immersion illuminator,

in plain substage fitting ... ... ... ... ......... ... £5 7 6

No. 3294. Ditto, in centring substage fitting ... ... ... ... ... ... 6 2 6

This illuminator can be accurately focussed for a slide of any thickness from \ m/m. to ljjl m/m. It has great advantage over the nonfocussing condenser, in that, where it is desired to examine already mounted specimens, the thickness of the slide upon which they are mounted is immaterial, provided it is between the above stated limits. In general routine work, the selection of slides for thickness is obviated.

The peculiarity of dark ground illumination is that the light is focussed to one definite position and unless this position coincides with the object, the latter will not be illuminated at all. As the illuminator must be in immersion contact with the lower surface of the slide, it cannot be moved up and down to alter its focus. For this reason a fixed focus dark ground illuminator can only be used with slides of a given thickness for which the illuminator has been made.

In the Beck focussing dark ground illuminator the optical portion consists of two lenses, C and D (Fig. 4), and the lower lens, D, can be moved up and down while the upper lens, C, remains in immersion contact with the slide B. By means of the lever projecting from the mount at the lower end (C, Fig. 5) the focus of the illuminator is adjusted for a slide of any thickness between £ m/m. and 1J m/m.

Fig. 4.

When this illuminator is used on slides varying from 1 m/m. to 1£ m/m. the cone of light which it transmits to the object is between 1 N.A. and 1-4 N.A., which enables any object glass with an aperture not higher than -95 N.A. to be employed with it. If, however, a slide is used that is not thicker than \ m/m., when the illuminator is focussed for this thickness the angle of the cone of light is altered so that its angle is between T25 N.A. and 1-4 N.A., and an object glass with an aperture of T2 N.A. can then be employed. We make a 2 m/m. and 3 m/m. both in the achromatic and apochromatic series, which while being equally useful for ordinary work is specially made for dark ground illumination.

The front lens of the illuminator can be unscrewed and then forms an excellent dark ground illuminator, which can be used dry and not in immersion contact with the slide, with low powers whose angle does not exceed •! N.A.

The substage fitting regularly supplied is made to the standard size T52 in. but special fittings can be supplied to suit other sizes at a small extra cost. In ordering for microscopes not of our manufacture this size should be given and also the distance from the bottom of the substage fitting when it is focussed as far up as possible to the top of the stage, so that the illuminator may be supplied of the correct height.

No. 3296. High power dark ground immersion illuminator, in plain substage fitting ... ... ... ... ... ... ... ........

No. 3297. Ditto, in centring substage fitting

£2 10 0 3 5 0

This illuminator throws upon the object a hollow cone of light between 1 and 1.3 N.A. It must be used in immersion contact with the slide and the slide must be 1 m/m. thick or the light will not be in focus.

It is specially intended for use with object glasses of high power, as it gives a very small but brilliantly illuminated spot of light. Oil immersion object glasses such as the 3 m/m. with an aperture of .95 can be used with it, but lenses with a greater aperture require to be stopped down by the insertion of a diaphragm behind the back lens.

No. 3482. Pinhole diaphragm for centring, in mount with standard R.M.S. object

glass thread............................................ £0 10 6

For particulars of special holders for dark ground illumination see page 99.

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