Text by Don Bruce, Images by William Ells
I suppose everyone interested in microscopy has a favourite microscope, I have always been attracted to miniaturisation and when it is combined with superb engineering mine has to be the McArthur microscope. (see photograph/s) This gem of a portable microscope is solid in construction all components being mounted in a aluminium block with a very firm mounting for the slide. The instrument measures only 4" x 2.5" x 2" (102 x 63 x 51 mm.) yet it is capable of all that can be accomplished with a bench microscope of similar optical specification, it will even accept a binocular head.
To keep within the limits of the small body Dr.John McArthur designed a folded optical prismatic system mounted in a detachable tube which approximated the 160mm. tube length of the standard microscope. This tube forms the base of the microscope. Light enters the instrument from above, either by means of the stainless steel mirror or from a built in electric lamp and passes downwards through the iris and condenser to the specimen. The objectives are below the specimen and the light forming image is reflected by the prismatic system to the eyepiece, the image is the right way up, not inverted as in a conventional microscope.
The microscope slide on which the specimen is mounted is placed coverglass down on the stage the upper surface of the cover glass being level with the upper surface of the stage. Thus the specimen is always in focus irrespective of the thickness of the slide. A fine focus control is provided to allow for specimen thickness, coarse focussing is unnecessary.
The three objectives are optically standard but mechanically shortened and mounted on a sliding plate, they are interchangeable, with a comprehensive range of objectives, including 4:1 and a 100:1 oil immersion, a phase contrast set for example could be mounted on a spare sliding plate, although I have only one plate fitted with 10:1, 20:1, & 40:1. dry objectives. Because the objectives are always in focus the Abbe condenser is also in permanent focus and likewise interchangeable. The eyepiece is a RMS size push fit and also interchangeable, mine is a 10:1.
The latest models are fitted with a tapping in the base which will accept a camera tripod. A short tripod greatly facilitates bench use of the microscope. Mine was an earlier model so I have manufactured a cradle to take the microscope and this can be fitted to a tripod. The weight of the microscope without its case is only 18 ounces (510 gms.). The one inconve- nience I have found is the standard 3" x 1" (76 x 25 mm.) microscope slide projects over the edges of the stage and can be easily caught by the fingers when the instrument is hand held and the objective plate is being manipulated.
The McArthur microscope has been manufactured by several companies since its inception. Today it is marketed by Prior Scientific Instruments, London Road, Bishops Stortford, Herts, CM23 5NB. Tel: 0127 506414. I do not know the current price, their 1980 price list quotes L407 plus VAT for the standard outfit for Bright field work, with mirror, Abbe condenser, Huygenian 10:1 eyepiece, 10:1, 40:1 & 100:1 o.i. objectives, lamp, simple light tube and vinyl case. At one stage Dr.McArthur manufactured the instrument himself and it is one from his production line which I own. I bought it second hand in mint condition in 1968 from Dr.Raymond Greene, brother of author Graham Greene, for L30. If you like the McArthur microscope but do not like its price it is sometimes available on the second hand market at around 250.00 pounds.
There is a very cheap alternative, and that is the McArthur microscope designed for the Open University, It employs the same system as the original microscope but is larger in length & height 5" x 3" (127 x 76 mm.) but at 1" (25 mm.) only half the thickness. It is made almost entirely of plastic with a total weight of 7.5 ounces (198.5 grams). The standard Open University microscope has a fixed 10:1 eyepiece with a sliding objective plate with fixed 8:1 & 20:1 objectives. It has an internal light source or it can be used with an external lamp or daylight. There is a tripod adapter in the base. The specimen slide is held longitudinally and is therefore more easily controlled and is not likely to be moved by the fingers when handling the instrument.
I have added a photograph of the Swift Field Microscope which I also own, it is similar in concept to the MacArthur but not so compact, for example the mirror and condenser is not built in. Note the objectives are on revolving holder not in line on a slider as on the MacArthur.
by Don Bruce 1995
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