of out Sensations. 269 



tion against which we ought to act from the commencement of 

 our studies, and which the professors called upon to direct the 

 first steps of a beginner in the sciences of observation should 

 use every endeavour to correct. Among the persons whom I 

 have succeeded in causing to see and feel what I wished and 

 not the external reality, there were naturalists, physicists, &c, 

 some of them having a name in science. It is certain that we 

 must always severely criticise our own observations, because 

 however able we may be, we do not perfectly know how to 

 make use of our organs, and we are too much inclined to give 

 them our confidence. 



The education received in laboratories diminishes the im- 

 perfections in question. If beginners in microscopic investi- 

 gations, for example, have to draw an object previously 

 described and which they have actually before their eyes, 

 they often represent it in accordance with the more or less 

 detailed description of it which has been given to them, and 

 not as it really exists. It is not at all uncommon, in the 

 case of a microscopic preparation, that the beginner declares 

 that he sees such or such a detail of structure of which he has 

 been told, and which the preparation does not show at all. 1 

 have ascertained this very frequently. And in this case there 

 is not always deception or falsehood on the part of the pupil, 

 who really sees what the master indicates to him; but he sees 

 it by a sort of hallucination of the psychical centres, and not 

 by a real impression striking his retina. Afterwards in the 

 course of study it is more rare to meet with any such pheno- 

 menon in those who devote themselves to observation ; but 

 nevertheless it is always necessary to be upon one's guard. 



It is also well known that our sensations are not always in 

 relation to the degree of intensity of the external excitation 

 (Werndt, Fechner, <&c). The expectation of a phenomenon 

 reduces to a minimum the degree of intensity of excitation 

 necessary to produce the phenomenon in order that it may be 

 followed by a sensation. "When the expectation is extra- 

 ordinarily vivid," says Mr. James Sully, "it may suffice to 

 produce as it were the simulacrum of a real sensation. This 

 is what occurs when the existing circumstances suggest to us 

 the idea of some immediate event. The effect is particularly 

 powerful in case the object or event expected is of a nature to 

 interest or excite, because then the mental image gains in 

 intensity by virtue of the emotional excitement which accom- 

 panies it"*. These conditions are met with in the facts which 

 we have just related. 



* James Sully, Les illusions des sens et de V esprit. A volume of the 

 International Scientific Library, Paris, 1882, p. 78. 



