546 



NATURE 



[January 22, 1920 



Perhaps the most significant and satisfactory 

 feature of the symposium is that it should have been 

 possible to attract for a meeting which extended from 

 2.30 to 10.15 so large an audience for the discussion 

 of the microscope and its applications to industry. It 

 is more than doubtful if such interest could have' been 

 aroused before the war. The optical industry of the 

 country, it is clear, will not fail to establish itself on 

 a secure footing for want of a market. If the home 

 products reach the necessary standard of perfection 

 and keep abreast of the advances which scientific 

 achievement in whatever field renders possible, the 

 reward is certain. This, we are convinced, needs 

 much more systematic investigation in advance of 

 immediate requirements than has been undertaken in 

 the past, greater readiness to be guided by scientific 

 principles rather than by tradition, and not least the 

 design of instruments with special reference to the 

 accuracy obtainable in the various manufacturing opera- 

 tions by the best machine tools. It is a hopeless enterprise 

 with one scientific adviser to attempt to compete with 

 another firm of similar size which employs twenty 

 such advisers. At present such assistance is difiicult 

 to obtain. It 4evolves upon our universities, no less 

 than upon our manufacturers, to consider where thev 

 stand, and to do their part towards the country's weil- 

 beiner by making optics a living subject rather than 

 resting satisfied with the knowledge of a hundred 

 years ago. Research on their part and on that of 

 other institutions is necessary ; the field is wide. 

 We look to them for that interest which we have every 

 right to expect. 



The afternoon session was preceded by an exhibition 

 of microscopes and auxiliary apparatus. The historical 

 collection of microscopes from the South Kensington 

 Museum was of special interest. New models of 

 microscopes attracted much attention. Messrs. Beck 

 and Swift exhibited models fitted with the changing 

 device they have adopted, and some exhibits bv Messrs. 

 W. Watson and Sons were greatly admired. Manv 

 other exhibits of much interest were shown, but for 

 particulars of these reference must be made to the 

 catalogue specially prepared for the occasion. 



The publication of the proceedings of the symposium 

 will be awaited with interest. We trust that all the 

 papers will be collected into a single volume, and 

 be available as a separate publication for all who have 

 special interests in microscopy. 



A 



CONSTRUCTION AND USE OF 

 MICROSCOPES.-^ 



CONSIDERATION of the microscope resolves 

 itself of necessity into two parts, the mechanical 

 and the optical. From the mechanical point of view- 

 there are two designs in general use, those referred 

 to as the Continental and the English forrri of micro- 

 scope. In the Continental type it has usually been 

 customary to have what is known as the horseshoe 

 foot, mainly, I imagine, because of its ease of con- 

 struction by mechanical engineering methods ; whereas 

 the English design of microscope, which has hitherto 

 been mainly made by hand, is of a more steady type, 

 and the points of support are so distributed as to give 

 more stability to the instrument in any position. 



The essential parts of the instrument are a coarse 

 adjustment, to give the body tube a quick motion in 

 the direction of the optic axis, and a fine adjustment, 

 which gives it a much slower motion in the same 

 direction. The tube is adjustable in length, to enable 



I Opening paper of a discussion on " Tiie Microscope: Its Design, 

 Construction, and Applications," organiscdiby the Faraday Society and held 

 at the Royal Society on January 14. By J. E. Barnard, president of the 

 Royal Microscopical Society. 



NO. 2621, VOL. 104] 



correction to be made for varying thicknesses of cover- 

 glass, although a large number of workers appear to 

 regard it as a ready method of obtaining greater or 

 less magnification, with disastrous effects on the 

 resulting image. 



There is only one fixed part of a microscope for 

 biological purposes, and that is the stage. But 

 metallographers require that the stage shall also be 

 adjustable in the direction of the optic axis. The 

 body tube itself should be made so that it can be 

 closed to a length of 140 mm., including any objective- 

 changing device that may be on the nose-piece ; and 

 it should be possible to lengthen it to at least 200 mm. 

 or 250 mm. if long-tube objectives are used. 



All these adjustments are in the direction of the 

 optic axis of the instrument. Two others are usually 

 provided which are at right angles to this direction — 

 that is, a mechanical stage for actuating the object, 

 and in certain of the best-class instruments an ar- 

 rangement for centring the sub-stage condenser to the 

 axis of the objective. 



While there are many points which might be raised 

 on the mechanical side, there are only one or two that 

 I have time to mention. The main point about most 

 microscopes appears to be that they are unstable. I 

 have a considerable number in mv own possession, 

 but I do not think I have one even now which, if I 

 centre an object on the stage with the instrument in 

 a vertical position, still maintains its centratioii 

 accurately if the instrument is put into the horizontal. 

 The probability is, therefore, that there are few micro- 

 scopes made at the present time that exactly fulfil the 

 conditions necessary for high-class photomicrographic 

 work or for observational microscopic work of an 

 exacting order. I trust, however, that an instrument 

 exhibited at this symposium will embody the neces- 

 sary improvements to rectify this matter. 



Some misapprehension appears to me also to exist 

 as to the relative purpose of the coarse and the fine 

 adjustments. The coarse adjustment appears to me 

 to be one which should be sufficiently well made, and 

 with which the user is sufficiently expert, to enable 

 him to bring into view any object, whether it is being 

 observed with a low- or a high-power objective. The 

 fine adjustment is then used for accurate focussing and 

 for getting a conception of the object in depth. In 

 biological work, at any rate, this is very rarely the 

 state of affairs as carried out. In using an oil- 

 immersion objective, for instance, a common method 

 is to immerse the objective, and then to lower it so that 

 it all but touches the top surface of the cover-glass. 

 The objective is then raised by means of the fine ad- 

 justment until the object comes into view. While this 

 may act fairly well with very thin cover-glasses, it is 

 a haphazard method when cover-glasses of varying 

 thicknesses are used. It should be realised that when 

 microscope-users are sufficiently educated they will be 

 able to tell how far they are from the actual image 

 by the appearance of the light in the field of view — that 

 is, if the object is illuminated with reasonable 

 accuracy. 



Mechanical stages also appear to need some con- 

 sideration. The stages which will on actuation cause 

 no shift of the object other than in the direction 

 intended, or any alteration of focus, are rare. 

 Further, those in which the screws project for a 

 considerable distance, with the result that any slight 

 jar or knock causes them to be displaced, and, it may 

 be, actually bent, are objectionable when used under 

 laboratory conditions. 



There is, I think, much to be said for the type of 

 stage which has either co-axial milled heads on a 

 vertical axis, or, if inconvenient to make, milled heads 

 which are on separate axes. This method of con- 



