40 THE PRESENT POSITION AND THE 



objective has of separating and forming correct images of fine detail. 

 That known as the Abbe Diffraction Theory, is the theory on 

 which modern optical calculations are based, and it is safe to say 

 that it was never more fully accepted and never rested on a 

 surer basis than at the present time. There has been much dis- 

 cussion in this country of that theory, and probably a good deal 

 of misconception has arisen from its inapt designation; for the 

 term " Diffraction Theory " is perhaps somewhat unfortunate. I 

 cannot do better than quote the late I-iord Rayleigh in refer.ence 

 to this matter. He said: " The special theory initiated by Professor 

 Abbe is usually called the Diffraction Theory, a nomenclature against 

 which it is necessary to protest. Whatever may be the view taken, 

 any theory of resolving power of optical instruments must be a 

 diffraction theory in a certain sense, so that the name is not dis- 

 tinctive. Diffraction is more naturally regarded as the obstacle 

 to fine definition, and not, as with some exponents of Professor 

 Abbe's theory, the machinery by which good definition is brought 

 about." 



This very clearly and accurately sums up the position. The 

 Abbe theory tells us that there are two main factors determining 

 resolution: that is, the numerical aperture of the objective used, 

 and the wave-length of the light. Numerical aperture is determined 

 for us by the optician, and it is well known that, with an oil- 

 immersion objective, a numerical aperture of 1.4 is at the present 

 time the practical limit. Metallographers are in a somewhat 

 stronger position, as a mono-bromide of naphthaline immersion 

 objective was, and presumably still is, made by Zeiss, which had 

 a numerical aperture of 1.6. This represents the absolute limit at 

 the present time, and there is no indication that numerical aper- 

 ture will be incneiased in this sense by present methods. 



The other factor governing resolution is the wave-length of 

 light, and in this connection it must be borne in mind that to 

 resolve a regularly marked structure, the distance between the 

 markings must be more than half a wave-length. Under ordinary 

 conditions of illumination we cannot go very far in the direction 

 of increased resolution unless we have resort to an illuminant such 

 as a mercury vapour lamp, which is rich in blue and violet radia- 

 tions. There is much room for investigation in this direction, as 

 the ideal ilkmiinant for microscopic work has 3/et to be found. But 

 I do not know of any one that approaches so nearly to it as the 

 one I have mentioned — the mercuiy vapour lamp. It suffers only 

 from one disadvantage that I can see, and that is that the differ- 

 entiation due to staining is not so clearly brought out as when 

 ordinary light is used. But as staining is itself an artificial process, 

 and is simply done to differentiate structures, it only means a certain 

 amount of education to enable us to appreciate the differences, even 

 under the light from this lamp. The only stains which it does 

 not show quite well, or, rather, in which the colour-tint is altered, 

 ai'e. tho?e in which red ])redoini nates. Anv other colour is shown 

 perfectly and in reasonable gradation. The advantages of this 

 illuminant are that it is even and uniform. It has a fairly large 

 area, and can be used therefore for any class of wcrk. It^ intensity 

 can be varied within considerable limits by having a resistance in 

 series, so that the current density is altered to suit the particular 



