332 



KNOWLEDGE. 



September, 1913. 



central line between the two pictures, as indicated. 

 Large prints may conveniently be hinged together by 

 a tape glued across the back at the junction, in order 

 that they may be kept unmixed and may fold up when 

 not in use. For prints over 8-in.x6-in. the stereo- 

 scope should be raised in the hand, so that a longer 

 viewing distance is obtained. Besides its ordinary 

 purpose the model illustrated is very suitable for 

 viewing stereoscopic illustrations in books. Another 

 pattern, it may be stated, makes provision for the 

 inspection of transparencies and autochromes. On 

 the whole, the Pigeon stereoscope — the last word, so 

 far, in reflecting instruments — certainly seems to 

 offer a maximum of efficiency, combined with 

 extreme simplicity. The image is exceedingly clear 

 and brilliant, while with a front lighting absolute 

 uniformity of illumination is easily secured. Those 

 who would like to have ocular demonstration of the 

 principle involved may readily do so by using an 

 ordinary plane mirror, about eight inches or ten inches 

 high, supporting it in a vertical position on the line 

 between the two prints, the silvered surface being to 

 the left, as shown by Figure 347. For the purpose 

 of experiment an admirable anatomical study from 

 an exhibit in Dijon Museum, taken by M. L. Chapuis 

 in collaboration with Professor Pigeon, and repro- 

 duced by kind permission of the latter, will be found 

 on page 329. The effect with an ordinary mirror 

 will not, of course, be so good as with a surface- 

 silvered one, or with the prism, but will still enable 

 an excellent idea to be obtained of the results possible. 

 It may be mentioned that an ophthalmological 

 stereoscope on the reflecting principle has been 



invented by Professor Pigeon, for the diagnosis and 

 treatment of strabismus, or squinting. Besides its 

 primary use, it may be employed for the direct study 

 of the problems of binocular vision ; for investigations 

 concerning the function of the motor muscles of the 

 eye and paralysis of these muscles ; for the physio- 

 logical fusion of colour ; for the medico legal 

 examination of alterations in sight, real or simulated; 

 and for many other purposes. 



In conclusion, some remarks concerning the 

 relative merits of small and of large stereographs 

 will not be irrelevant. It has been stated by several 

 writers that a small print inspected at a short focal 

 distance gives really the same stereoscopic effect as 

 a larger print viewed from a greater distance. To a 

 certain extent this is true ; but there is a grave fallacy 

 in the reasoning, since the much greater amount of 

 detail secured in the larger picture is overlooked. 

 To make the matter plainer, compare the case of an 

 enlargement made from a small negative with that 

 of a large direct photograph the same size as the 

 enlargement. There can be no question as to which 

 of the two will give the more detail. The large 

 direct print will show many things that are not 

 visible in the enlargement, it being, therefore, clear 

 that they are absent in the small picture. Large 

 prints, moreover, are seen with less fatigue, make a 

 more definite impression, need no objectionable 

 magnification, may be made with lenses of a more 

 acceptable focal length, and are more useful for 

 purposes of investigation or for the deduction of 

 measurements. 



BEDROCK: 



In the current number of Bedrock the " Hermit of 

 Prague " makes fun of the Headmaster of Eton for 

 taking seriously Miss Curtis's New Mysticism, the 

 nature of which she explains in her book " Medita- 

 tion and Health." This the " Hermit of Prague " 

 describes as one of the numerous wholly unscientific 

 works on " mind-healing " that for several decades 

 have gained a curious vogue in the United States. 



Under the title of " Mendelism, Mutation and 

 Mimicry," Professor Punnett very clearly sets forth 

 what in his mind are the leading difficulties against 

 accepting the theory of mimicry as explaining the 

 likeness between various butterflies. He charac- 

 terises as an enormous assumption Professor 

 Poulton's belief that a very slight accidental 

 variation on the part of a species in the direction 

 of a pattern which is utterly different, will be 

 detected by its enemies and cause them to let it 

 alone. He asks how an enemy endowed with such 

 remarkable discrimination could fail to distinguish 

 between mimic and model even in cases of the 

 closest resemblance yet recorded. 



The evidence in favour of the existence of pre- 

 palaeolithic man is brought forward by Mr. J. Reid 



Moir, and he sums up his arguments by saying that 

 there is one thing, and one thing only, left for the 

 opponents of the pre-palaeolithic implements to do 

 if they wish their views to be taken seriously, and 

 that is to subject flints to some unguided, natural 

 force, and produce forms indistinguishable from 

 those which are in dispute. 



Mr. Hugh S. Elliot, . commenting upon Dr. 

 McDougalPs article in the previous number of 

 Bedrock, says that while historically and scientific- 

 ally Dr. McDougall is justified in applying the name 

 " Materialism," to his (Mr. Elliot's) published views, 

 the word carries with it in the public mind many 

 connotations which are very far from the opinions 

 of those who profess it. The name " Scientific 

 Materialism " is suggested instead and used as a 

 title for the article, which is an answer to Dr. 

 McDougall. 



Among the contents of the same number is a 

 contribution by " A Business Man," which he calls 

 " The Truth about Telepathy." The writer grumbles 

 because Sir Oliver Lodge attributes motives to him, 

 but we cannot say that we admire the methods of 

 " A Business Man." 



♦ For July, 1913. 



