The Britex Microscope

A look at an older 'scope used in schools

By Paul James (uk)

 
I must confess that the name 'Britex' always conjured up imagery of toy microscopes, despite the genuine intention of the manufacturer to fuse the words 'British' and 'technical' into a contemporary logo of the era. Toy microscopes abound, and vary within a limited range of optical quality, but this 'Britex' microscope is no toy. It has the stature, mass and functionality of a mid 20th Century stand, though lacking in some mechanical refinements. This instrument was capable of taking a young student through to university, where he or she could utilize and enjoy the fineries and revelations of a more sophisticated microscope.

General mechanical features

The images show a simple yet solid stand with grey speckle finish and weighing in at just over 3 kilo's, so though it is no heavyweight, it is solid and stable enough, yet not too difficult for young students to handle from case to benchtop. It's simplicity belies its mechanical quality, and apart from some minor niggles, which have more to do with its age than design faults, the stand is very well made. It is clearly a simple yet reliable instrument made for use in schools, and the fact that it has survived about 25 years in this environment is evidence of good design and manufacture.

A plain foot is used which is very compact and neat, which holds two adjustable screws set to control the limb's vertical and horizontal extremes. The 'pin' holding it to the limb is solid yet easily adjusted to control friction.

Simple elegance would be the way I'd describe the limb with its nice sweep, affording uncomplicated handling which is important in a school environment, where safe passage across the lab is essential!

Composite material is used for the stage which has stood the test of time too, and is still at right angles to the axis! The two stage clips show understandable wear and tear, but they were easily brought back to their original state.

Substage 

Mechanically speaking the placement and housing of the substage condenser is simple, though lacking a centering cage. This latter feature is desirable in all but the most primitive class of instrument. The focussing of the condenser proved difficult because it needed freeing as well as adjustment at first, being of the simple helical form. I think this was the only drawback of the microscope's operation, but certainly easy enough to put right with some care.

The iris is accurately centred, but I found difficulty in locating its operating lever since that depended on the setting of the condenser's focus, which in turn varied its peripheral position by up to about about 60-70 degrees, owing to its helix mounting. Not the sort of feature a child could automatically compensate for easily, let alone an adult used to operating microscopes. 

The mirror is in excellent condition as can be seen, and has both flat and concave faces of good size. It can be removed from the limb if necessary to enable horizontal photography with external illumination.

Focussing mechanisms 

Here the traditional coarse focus mechanism is used, though no dovetail slides are present. Instead the tube is housed in a cylindrical matched portion of the upper limb. It operates smoothly through its rack and pinion and can focus its x40 objective reassuringly. There was no backlash or side float with the coarse focus controls, and its entire travel was adequately smooth. A centre screw can be tightened if deemed necessary to increase the resistance of the coarse focus, and a simple, but worthwhile 'stop ring' is provided in the form of a rebate just below the eyepiece to limit its maximum descent. This potential hazard of damaging objective front lenses and important slides still exists on some modern microscopes, so it was satisfying to see a 'belt and braces' approach to this problem.

The fine focus is unusual in that the lower end of the 'draw' tube can be raised or lowered by rotating a knurled ring. In this example it was heavily greased which reduced its sensitivity, but was easily remedied. It was at this juncture that I realised the general engineering of the microscope was of a very good standard. Finding this knurled ring whilst peeping through the eyepiece took some getting used to as my hand was searching for a more conventional control.

The Optics

I was impressed by the optical quality of this microscope. Even on difficult subject matter, the images were easily mistaken for those from standard biological instruments. Uniformity of the illuminated field was good on all powers and showed fairly good resolution right across it. The 0.7NA x40's performance was in excess of what would have been required from its use in schools, and the 0.25NA x10 also. Field flatness was also good, using the x10 Huygens eyepiece provided, though eye relief was minimal. The third objective was missing, so I have placed a x3 Baker optic there which is nearly parfocal with the other two and complements them satisfactorily. 

No pinhole apertures here.
The Abbe condenser was well made and though unmarked, appeared to have an aperture in excess of 1.0 NA, and therefore more than sufficient for the needs of the x40 objective. Pity then that its focussing was less than ideal. This raised a thought about what sort of objective might have occupied the third place on the changer. Noticing the provisional holes in the stage for a mechanical stage attachment suggested, together with the highish aperture of the condenser that an oil immersion lens might have been included. But I cannot imagine an oil lens to be standard kit for a student in a school, so more likely the missing optic was a lower power like the x3 I attached ?

Although only one x10 Huygenian eyepiece came with this instrument, its case showed evidence of homes for 2 more. All together a tidy optical outfit.

Conclusion

Despite a couple of minor niggles, which could be easily remedied, this instrument remains a well conceived and engineered example of that class of microscope which could be found in large numbers in schools in the UK about 25 years back. Coupled with the mechanical stage, which was missing, this microscope would have been certainly capable of keeping a keen student of biology busy for a number of years. This model is anything but a toy...........my apologies 'Britex' !

 
All comments to the author Paul James are welcomed

 

Microscopy UK Front Page
Micscape Magazine
Article Library

© Microscopy UK or their contributors.
Published in the June 2003 edition of Micscape Magazine.

Please report any Web problems or offer general comments to the Micscape Editor.

© Onview.net Ltd, Microscopy-UK, and all contributors 1995 onwards. All rights reserved. Main site is at www.microscopy-uk.org.uk with full mirror at www.microscopy-uk.net.