192 DIFFRACTION-PHENOMENA IN FILMS OF BLOOD CELLS. 



It was seen that suitable films of this nature gave rise to 

 diffraction-phenomena, consisting- of circular spectra, measuring 

 from about 5 to about 30 centimetres. 



The relation between the diameters of these spectral rings 

 and the diameters of the bodies composing the " grating " was 

 discussed, and evidence was brought forward to uphold the 

 theory that the diameter of the spectral rings was a function of 

 the diameter of the bodies constituting the " grating." It was 

 pointed out that this circumstance provided the observer with a 

 neiv method of measuring the diameter of " round " bodies of 

 microscopical size, and stress was laid on the fact that this new 

 method is superior to the old method of taking microscopical 

 measurements for the following reasons : — 



(i) The dimensions actually to be measured (diameter of 

 spectral rings) surpass those to be calculated (diameter of bodies 

 in grating) so many times, that the method becomes sufficiently 

 sensitive to register differences in size zvliich have to be expressed 

 in hundredth parts of a nvicron. ■ 



(2) As all the bodies which are struck by the beam of light 

 contribute towards the formation of the dififraction-phenomenon, 

 the method gives tlxe average diameter of millions of bodies by 

 means of one measurement and one caiculdiion only. 



A graph was shown which illustrated that the apparatus had 

 made it possible to record changes in diameter occurring in red 

 blood cells under the influence of very slight changes in osmotic 

 pressure of the fluid surrounding them, and by the addition of 

 minute quantities of specific haemolytic amboceptor. 



A striking difference in diameter of the spectral rings, as 

 produced by means of blood cells of human beings and those of 

 sheep, was readily made visible by the apparatus. 



In conclusion, a series of surface-cultures of micro- 

 organisms, all of different sizes, were placed on the stage of the 

 apparatus, and a marked difference in diameter of the spectral 

 rings produced by them could be observed in every instance. 



For further particulars as regards the apparatus, its appli- 

 cations and all details of the method, the reader is referred to 

 The South African Medical Journal for August, 1918, and June, 

 1919. 



