Dr. G. J. Stoney on Microscopic Vision. 345 



The chief (though not quite the only) effect of this is simply 

 to render the image incapable of exhibiting some very fine 

 detail upon the object which before it was able to reach. But 

 the addition of — Bb has a worse effect. It adds io the image 

 an entirely new set of fine rulings which do not represent any 

 of the features which exist upon the object, and by this light 

 such false effects as intercostal markings, spurious resolutions, 

 a general haze of light, &c, are apt to be, and often are, pro- 

 duced. Hence we may enunciate Proposition 3 as follows: — 



Proposition 3. 

 When, of the light emitted by the object, only part is employ (d 

 to form the microscopic image, then features may intrude them- 

 selves into the microscopic image which are not present in the 

 standard image, and which do not represent anything upon the 

 object. 



16. Proposition 4. False Colouration. — Another deceptive 

 effect which is to be referred to the limited apertures of 

 objectives is the appearance given to uncoloured objects of 

 being coloured. Only the general principle to be kept in 

 view will be stated here, as a fuller treatment of this phe- 

 nomenon can be more conveniently made in connexion with 

 individual instances which will be dealt with in Part II. of 

 this memoir. 



The whole light of wave-length \ which is sent forward by 

 the object may be divided into A A which is admitted to the 

 objective, and B A which is excluded. A similar partition 

 into these two portions is to be made of the whole light 

 of each wave-length, but the proportion in which the whole 

 light is divided between them in general varies from one 

 wave-length to another. Hence, if the illumination is by 

 white light and the object uncoloured, there may be a pre- 

 ponderance of light of some colours in A as compared with 

 others, and an equal deficiency of these same colours in B. 

 In such cases the image seen in the microscope, since it is 

 exclusively formed by the light A, has not got the colours 

 mixed in the same proportions as they are in white light, and 

 accordingly appears coloured. Hence 



Proposition 4. 

 Under the same circumstances as in Proposition 3, the par- 

 tition of the light beticeen the portions received by a7id excluded 

 from the objective, will in general be different for different wave- 

 lengths ; and ivhen the difference is marked a colourless object 

 will appear to be coloured in the microscope. 



