Beck Object Glasses. 
The manufacture of object glasses has advanced in recent years. By the invention 
of new formulae and by improved methods of testing we may claim to be producing object 
glasses which are somewhat in advance of anything hitherto made in this or any other country. 
In ordinary object glasses, two series are made, achromatic and apochromatic. The achromatic 
series fill the requirements for most purposes, and are in more general use on account of the fact 
that they are made to a simpler formula and of less expensive materials and are consequently able 
to be sold at a lower price. The resolution is of a high order and is, in all lenses, as near to the 
theoretical resolving power as is attainable. The chief feature of the apochromatic series is that 
different glass is employed and other materials substituted and this combined with 
a different formula involving the use of a larger number of component lenses produces an object 
glass in which there is a more perfect correction for chromatic and zonal aberrations. In achro- 
matic object glasses the correction is made for two colours of the spectrum but in apochromatic 
lenses the correction is made for three colours. For very fine structure undoubtedly apochromatic 
lenses give superior results; the perfection of the colour correction enables certain objects to be 
seen with greater crispness than is possible with achromatic lenses and higher power eyepieces 
can be used without breaking down the image. For photography the apochromatics have a 
decided advantage. 
All object glasses, unless otherwise ordered, are corrected for a tube length of 160 m/m., 
and the drawtube should be extended to the 160 mark. The Sloan objective changer measures 
10 m/m., and if this is used the tube should be extended to the 150 m/m. mark. Similarly if a 
revolving nosepiece is used which measures 10 m/m., the drawtube should only be extended 
to the 150 m/m. mark. 
Dry object glasses, unless otherwise ordered, are adjusted for use with a coverglass -006 inch 
(15 m/m.) thick, and a small thickness of mounting fluid, and coverglass of approximately this 
thickness should be used for all high power work. The exact thickness of coverglass to be employed 
for a dry object should be -007 inch (-18 m/m.) with the tube length of 160 m/m. or -006 inch 
(15 m/m.) with tube length of 180 m/m. Object glasses specially ordered for use with uncovered 
objects such as metallurgical specimens are supplied at the same prices, unless short mounted, 
when a small extra charge is made. 
All achromatic object glasses, except the lowest powers, 32 m/m. to 60 m/m. focus, have 
the same optical lengths, so that when used on a Sloan objective changer or a revolving nosepiece, 
they are in focus without more than a small movement of the fine adjustment. 
For dark ground illumination it is often the practice to place a removable stop in the back of 
the object glass to reduce its aperture to the requisite amount to exclude direct light. This method 
has the disadvantage that the stop, not being in the most effective position for its purpose, it is 
necessary to cut down the aperture to a greater extent than is actually required, thus sacrificing 
the resolving power of the object glass. The ~ inch (2 m/m.) and } inch (3 m/m.) 1:2 N.A. 
apochromatic and achromatic object glasses are specially made for use with our high power 
focussing dark ground illuminator with } m/m. slides, the full aperture being employed. For thicker 
slides up to 1} m/m. thick a stop is provided to reduce the aperture to :95 N.A. The } inch (3 m/m.) 
N.A. -95, and the # inch (2m/m.) N.A.1, achromatic object glasses are suitable for use with any 
type of high power dark ground illuminator with a slide of approximately 1 m/m. thick, their full 
aperture being employed. All these object glasses are also suitable for use with direct illumination. 
Object glasses to be used with the Beck aplanatic ring illuminator are supplied without 
extra cost specially mounted to allow of the full illuminating angle being utilised but lower powers 
than 16 m/m. require a special adapter. 
Monochromatic object glasses are also made, one specially for metallurgical purposes with 
the exceptionally large aperture of N.A. 1-6, also a series is made for use with ultra violet light. 
These will be found described on the following page. These object glasses have been found most 
satisfactory in use by those who employ them for their special purposes and very successful work 
has been carried out with them. 
For hints on the use and selection of object glasses see “‘ The Microscope, a simple handbook,”’ 
and for further details of illumination and technique see “‘ The Microscope, an advanced handbook.” 
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