304 THE SCIENCE OF MICROSCOPY. 



enquiries whose aim is to determine material facts, microscopy 

 needs to verify its results by special processes of induction, as well 

 as by critical supervision of its methods, it is weighted with an 

 additional burden of precautions, conditioned by the circumstance 

 that actual objective certainty is here only attainable by a reasoning 

 observation substituted for direct sight of the object. 



Everybody will probably admit that when an object is removed 

 from direct observation, and can only be seen at second hand and 

 interpreted by signs, that is to say, when nothing but a light and 

 shadow image of it is presented to the eye, deceptions of the visual 

 sense are more likely to occur than with objects which can be 

 examined with the natural eye. The intervention of a complicated 

 image-forming lens apparatus between the eye and the object to be 

 inspected, necessarily introduces considerations of optical law in 

 determining what is seen and what it means, as also what 

 manipulation of the instrument, and what illumination of the 

 object will enable the microscopist to realise in practice all that 

 theory indicates to be possible. If therefore, under ordinary 

 circumstances^ the art of observing needs discipline and culture to 

 free itself from numberless sources of illusion,* it should follow 

 that observation though the microscope must be cmteris paribus, 

 more Hable to error both of sight and reason. And while fallacies 



* In describing the theory of vision taught by Epicurus, Lucretius (Essay 

 on the Nature of Things) makes the following remarks. — " It is not the eyes 

 "that cheat us: it is the mind which makes wrong inferences from the data 

 ''given by sight. It is the province of the eyes to observe on what spot 

 ' • soever light and shade are : but whether the lights are still the same or not, 

 "and whether it is the same shadow which was in this spot that is now 

 "passing to that, t/iis, the mind only has to determine: nor can the eyes 

 " know the state of things. Light and shade— it is this and this alone we 

 " can really be said to see. Distance and solidity — in a word the real figure 

 " and the real position of anything, this we do not see but infer. And if we 

 " would seek for the source of optical delusions, we must seek it only in * the 

 " mental suppositions which we add of ourselves, taking those things as seen 

 " which we do not see. For nothing is harder than to separate manifest facts 

 'from doubtful ones ivhich the mind straightwai/ adds on of itself ' " 



