No. 3293. The Beck patent focussing dark ground immersion illuminator, 
in plain substage fitting ... Bt So: ene = Sn re hw 
No. 3294. Ditto, in centring substage fitting Se = ao die sae ra 6 2 6 
This illuminator can be accurately focussed for a slide of any thick- 
ness from 4 m/m. to 14 m/m._ It has great advantage over the non- 
focussing 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 4 m/m. and 1} m/m. 
eg | When this illuminator is used on slides varying from 1 m/m. to 14 m/m. the 
PA i i ‘ = 7s z A yy, 5 2 
VT: 1 cone of light which it transmits to the object is between 1 N.A. and 1-4 N.A., 
jsut which enables any object glass with an aperture not higher than -95 N.A. to 
be employed withit. If, however, a slide is used that is not thicker than 4 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 1-25 N.A. and 1-4 N.A., and an 
object glass with an aperture of 1-2 N.A. can then be employed. We makea 2 m/m. 
and 3 m/m. both in the achromatic and apochromatic series, which while being 
Cc equally useful for ordinary work is specially made for dark ground illumination. 
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The front lens of the illuminator can be unscrewed and then forms an excel- 
lent dark ground illuminator, which can be used dry and not in immersion 
contact with the slide, with low powers whose angle does not exceed :7 N.A. 
The substage fitting regularly supplied is made to the standard size 1°52in. 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 -..: mae ats ce Kid pe ae She ve oe ZORRO 
No. 3297. Ditto, in centring substage fitting ee noe ee a: as za 30) 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 
spotof light. Oilimmersion 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 
No. 3297. behind the back lens. 
No. 3482. Pinhole diaphragm for centring,in mount with standard R.M.S. object 
glass thread ... os suk ae ure =o Sor O50 Tee O LOG 
For particulars of special holders for dark ground illumination see page 99. 
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