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POPULAR SCIENCE REVIEW. 
and not acted on as if real, the person subject to them is considered to he 
sane ; when the illusion, however, is believed to have a real and positive 
existence, and this belief is not removed either by reflection or an appeal to 
the other senses, the individual is said to labour under a delusion. These 
distinctions, so well known to psychologists, are scarcely at all recognized 
as yet by the popular mind ; and it is only through the instrumentality of 
works such as that which we have now under review, that we can hope to 
make them more generally understood. 
As expressed by the author, ‘ Common sense knowing nothing of fine 
distinctions, is wont to draw a sharp line between the region of illusion and 
that of sane intelligence. To be the victim of an illusion is, in the popular 
judgment, to be excluded from the category of rational men.’ Yet it must 
be granted 1 that most men are sometimes liable to illusion.’ 1 A momentary 
fatigue of the nerves, a little mental excitement, a relaxation of the efforts 
of attention by which we continually take our bearings with respect to the 
real world about us, will produce just the same kind of confusion of reality 
and phantasms which we observe in the insane.’ The term is used by the 
author in a wider sense than is usual with psychologists, and made to 
‘ include under it errors which do not counterfeit actual perceptions.’ For 
instance, by this popular extension of the term, a man is supposed to be 
1 under an illusion respecting himself when he has a ridiculously exaggerated 
view of his own importance and in a similar way of being ‘ in a state of 
illusion with respect to the past, when through frailty of memory he 
pictures it quite otherwise than it is certainly known to have been.’ 
The distinction between illusion and hallucination, or the sensations 
which are supposed to be produced by external impressions, although no 
material objects act upon the senses at the time, is not considered by the 
author to be very important ; since in ‘ many cases it must be left an open 
question whether the error is to be classed as an illusion or as a hallucina- 
tion.’ Still, and we think wisely, ‘ the distinction is kept in view, and 
illustrated as far as possible.’ Illusions are classified under the following 
heads : — Of Perception, Dreams, Illusions of Introspection, Errors of Insight, 
Illusions of Memory, and of Belief. The reader cannot fail to meet with 
much to interest him under each of the above heads. In the last chapter, 
entitled ‘ Results,’ we are told that the study of illusions ‘ has tended to 
bring home to the mind the wide range of the illusory and unreal in our 
intellectual life ; ’ but it is satisfactory to find further on that 1 no doubt its 
grosser forms manifest themselves most conspicuously in the undisciplined 
mind of the savage and the rustic.’ 
A DICTIONARY OF CHEMISTRY.* 
T HE first volume of this work was published in 1 863, or about eighteen 
years ago, and the concluding part of the Third Supplement now 
* A Dictionary of Chemistry and the Allied Branches of other Sciences. 
Third Supplement. Yol. III., Part 2. By Henry Watts. Longmans & Co. 
