1905.] High Poiver Microscopy 



WEEKLY EVENING MEETING.' 

 Friday, Febmaiy 17, 1905. 



Sir James Crichton-Browne, M.D. LL.D. F.R. 



and Vice-President, in the Chair. 



John W. Gordox, Esq., M.R.I. 



High Poiver Microsco^nj. 



In the exhibition of a microscopic object under high magnifying 

 power, there are three stages at which difficulties have to be en- 

 countered and surmounted. 



1. In the preparation of the object for exhibition under suitable 

 conditions of illumination. 



2. In the representation of the object by means of an image. 



3. In the transmission of the image so formed in the instrument 

 to the eye of the observer. 



Dealing first with the preparation of the object. Professor 

 Wright has suggested a classification from this point of view accord- 

 ing to which microscopic pictures faU into two classes, which, 

 adopting a nomenclatm-e employed by Professor Koch, he calls 

 colour pictures and outUne pictures. A colour picture, as its name 

 suggests, is usually the result of a stain, but its specific character does 

 not depend upon its tint. The distinctive property of a colour 

 picture is that the structure is shown by masses or washes of colour 

 without delineation, whereas in the outHne picture the contours are 

 dehneated and the masses have the same tone as the background. 

 Fig. 1 — a photograph of a piece of lung tissue — is an example of a 

 colour picture. Fig. 2 — a photograph of four strands of gossamer 

 — affords an illustration of an outline picture. The colour picture 

 possesses many advantages, especially where measurements are in 

 question, for the single boundary which is the coromon outline 

 both of the object and of the background, is more easily identified 

 than the more or less nebulous hne by means of which an outhne 

 picture is dehneated. But with a large number of objects the stain- 

 ing method fails altogether, and it is necessary therefore to have 

 recourse to the alternative type of picture. This depends for its 

 formation upon difference of refractive index between the object and 

 the background. The theory is perfectly well known, but may be 

 usefully brought to mind by a simple illustration such as is afforded 

 by Fig. 3. A very close and easily intelhgible analogy to the theory 

 of refraction may be found if we assume first that Fig. 3 illustrates 



