THE ULTRA-MICROSCOPE AND ITS APPLICATION 

 TO THE STUDY OF COLLOIDS. 



By E. JOBLIXG. 



It is often imagined, by those unacquainted with 

 either of the sciences of hght and microscopy, that 

 it is merely a question of difiiculties in construction 

 of the microscope, jjarticularly of the lenses employed, 

 which prevents us from perceiving the verj- smallest 

 of objects b\- its aid. Such, however, is not the 

 case. At certain definite limits of size, phenomena 

 intervene which effectually mar every effort at the 

 microscoping of objects with diameter below this 

 limit. The root of the difficulty lies in the phe- 

 nomenon of light itself. The fact that light is a 

 transverse vibration of appreciable wave length is of 

 slight importance when dealing with the common 

 objects of everydav' experience, but immediately the 

 attention is turned to objects of size comparable with 

 this wave length, the mechanism of light propagation 

 assumes tremendous significance. Then it is that 

 the ordinary laws of reflection and refraction are 

 waived in favour of the more subtle, and therefore 

 the more interesting, phenomena of diffraction. 



On the commonsense principle that a sea-wave is 

 but little disturbed by a pebble, though profoundly 

 modified when brought up against a sea-wall, it 

 seems reasonable to assert, without further explana- 

 tion, that an object comparable in size with a wave- 

 length of light cannot be expected to reflect waves 

 which could convey to the eye a distinct impression 

 of its size and shape. True, it does affect a minute 

 disturbance, but of this more anon. Without 

 venturing into a mathematical discussion of a 

 microscope's " resolving power," it will be sufficient 

 to state that one cannot hope to perceive an object 

 whose size lies below one quarter of a wavelength 

 of light, this constituting the limit above referred to. 



However, fortunately or unfortunately, scientists 

 have interest in objects much smaller than these. 

 Thus, the ever-increasing subject of colloidsdemanded 

 an instrument for the direct observation of colloidal 

 particles: and bacteriology, again, found itself ham- 

 pered for want of a means of visualising the cells 

 and microbes with which it is concerned. The 

 difficulty has been happily surmounted by the 

 introduction of the ultramicroscope, an instrument 

 which, as its name implies, renders it possible to 

 demonstrate to the eye the existence of objects 

 invisible ordinarily tothe finest microscope. 



This does not imply that the ideas put forward as 

 to the limits of optical resolution have been in the 

 least contradicted, as the following consideration 

 will show. To be seen, an object must either reflect 

 the light by which we see it or be self-luminous. 

 Consider only a very small particle. If it be of a 

 magnitude above the optical limit, it may be seen by 

 virtue of the light it reflects, and its true form can 



then he perceived. If, however, its magnitude be 

 below this limit, reflection is impossible, and all 

 incident light becomes diffracted, i.e., scattered in all 

 directions. In the latter case, the object emits light 

 exactly as though it were luminous of itself, though 

 under ordinary circumstances it remains invisible 

 because the [iroportion of light reaching the eye 

 with regard to the incident light is so small. If now 

 means be adopted for concentrating light upon the 

 object, it might be possible, other circumstances 

 being favourable, to make it emit sufficient light to 

 reach visibility. Of course, the object then cannot 

 be seen in the sense that its shape or surface can be 

 observed, but onlj- seen in the sense that its presence 

 is indicated by the light it emits, being observed in 

 the microscope as a minute disc of light. The 

 utilisation of such a concentrated illumination is the 

 fundamental idea underlying the construction of the 

 ultramicroscope. 



This principle was foreshadow ed b\- a phenomenon 

 observed long ago by Tyndall, who passed an intense 

 beam of light through the air of an otherwise 

 darkened chamber, w hen the track of light was made 

 clearly visible by the light diffracted from the minute 

 dust-motes floating in the atmosphere. Ultra- 

 microscopy furnishes an extension of this, a 

 microscope being introduced for the fuller examina- 

 tion of the track of light. Thus, Zsigmondy, in 1900, 

 as part of his obser\ations on some colloidal gold 

 solutions, reflected a cone of light into the solution 

 and examined the apex of the cone through a 

 microscope. Working on the same princijjle, 

 Siedentopf was led in 1903 to the improvement of 

 the apparatus, with the result that we owe to him 

 the introduction of the modern high-power 

 illuminating arrangements which form the main 

 feature of the instrument. 



Naturalh', certain conditions require to be fulfilled. 

 In the first place, the most intense illumination 

 possible is necessary to render the smaller particles 

 visible : secondly, to prevent dazzling of the eye 

 and consequent inability to detect the minute light- 

 discs, no illuminating ray must be allowed to fall 

 upon the eye either directly or by reflection ; thirdl}-, 

 in view of the often extreme faintness of the light- 

 discs, the darkest possible background is essential ; 

 and lastly, the beam of light must be extremely 

 shallow in the direction of the line of sight, else 

 nothing more definite would be observed than a 

 luminous haze. Only by rigid satisfaction of the.se 

 conditions can definite results be obtained. 



In the light of the above requirements, the following 

 diagrammatic arrangement of the apparatus con- 

 stituting one of the most delicate forms of the ultra- 



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