No. 3644. Dark ground illuminator for low and moderate powers ... re EDs On0 
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No. 3644. 
This illuminator is for use with object glasses the 
numerical aperture of which is not greater than °65 N.A. 
It is made somewhat on the principle of an ordinary 
substage condenser but has a specially long working 
distance. It is provided with an iris diaphragm for 
transmitted light in the usual manner. When the iris 
diaphragm is opened to its fullest extent and the tray 
is swung aside, a special stop can be folded in, which 
converts the apparatus into a dark ground illuminator. 
A neutral glass screen is supphed to fit into the tray, in 
order to reduce the necessarily strong light required for 
dark ground illumination to an amount suitable for 
ordinary transmitted light. With this illuminator, dark ground illumination can be used with 
all powers up to § inch 
(4 m/m) provided the object glass has not an aperture exceeding N.A. °65. 
No. 3357. Spot lens for low power dark field 
illumination, mounted to fit standard substage £1 0 0 
No. 3359. Beck aplanatic ring illuminator 
for the illumination of opaque objects with 
powers of 1din., $in. and 4in. The mounts of 
the object glasses must be specially arranged 
to take the illuminator. The illuminator consists 
of a silvered glass ring of such curves that 
light thrown upwards from the flat mirror is 
condensed to a very perfect aplanatic focus. 
The illumination is exceedingly brilliant and is 
directed upon the object from all sides. 210 0 
No.3359A. Beck aplanatic ring illuminator 
for 4in. object glass ae ae aD ZLOB TO 
No.3365.—Immersion ring illuminator for oi] immersion object 
glasses with apertures up to 1.3 N.A. For this purpose a special immersion 
object glass is made in which the whole front mounting is made of glass 
instead of meta]. The glass mounting has an outer surface which is 
spherical in shape and concentric with the focus of the object glass so 
that the light from the aplanatic ring illuminator passes through without 
refraction. The glass front mount has a small aperture in its centre and 
into this aperture the front lens of the object glass is cemented, so that 
the portion of the front lens usually employed for mounting it into a metal 
cell is used for transmitting light to the object. The front surface of the 
front lens of the obiect elass is level with the front surface of the glass 
mount and the whole of this surface is in immersion contact with the 
object when in use 
No. 3365. 
No. 3365. Immersion ring illuminator with 3 m/m. apochromatic object glass 
eo NEA : £20 OR 10 
No. 3365A. Ditto but with 3 m/m achromatic object glass 1.3 N.A. Banc ee 126 
No. 3367 Dark well for use with ring illuminators. This is mounted in a 
ring of the standard substage size and the well has an adjustment 
up and down and thus provides a suitable stop for obscuring the 
central beam of light from the mirror ... oe Be 2d su © 
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