November 18, 1904.] 



SCIENCE. 



683 



VAEI^ AUCTOEITATIS. 



To THE Editor op Science: Mr. Emmons, 

 in Science (October 21, p. 537), gives Pro- 

 fessor K. von Zittel's ' History of Geology and 

 Paleontology,' p. 3, as his authority for the 

 statement that ' Origenes reports of Xenoph- 

 anes of Colophon that he had observed sea- 

 shells on mountains, etc' But Eitter and 

 Preller, ' Historia Philosophise,' §140, a (p. 

 86), are more correct in attributing the state- 

 ment to ' Hippolytus, Ref. Heer., I., 14.' The 

 ' Philosophoumena, or Adversus omnes 

 hsreses,' attributed formerly to Origenes, 

 was proved by Bunsen in his ' Hippolytus and 

 his Age ' to have been the v^ork of the latter. 

 See Donaldson's ' History of the Literature 

 of Ancient Greece,' Vol. II., p. 323, n. 1. 



Heney W. Haynes. 



Boston, October 29, 1904. 



SPECIAL ARTICLES. 

 AN OVERLOOKED FORM OF STEREOSCOPE. 



Modifications of instruments, though in 

 themselves not important, are often of in- 

 terest as illustrating the variety of ways in 

 which a given principle may be expressed 

 in practise. This is notably true of the 

 stereoscope, which as a practical instrument 

 may be defined as any device that gives to 

 each eye its appropriately different view and 

 then enables the eyes to combine two views 

 with facility. The oldest form of the appa- 

 ratus, as is well known, was devised by Sir 

 Charles Wheatstone in the year 1838, and con- 

 sisted of two mirrors set nearly at right 

 angles and of two separate and appropriately 

 different views of the object (in the early ex- 

 periments always two mathematically con- 

 structed diagrams) carried at the ends of two 

 movable frames." The serious disadvantage 

 of this apparatus was noticed by the inventor 

 himself and consisted in the fact that the two 

 views, being separated, required a trouble- 

 some adjusting to secure an exact combina- 

 tion of their images. A great improvement 

 introduced in the present form of the appa- 

 ratus, which was due to Sir David Brewster, 

 was that the two views could be permanently 

 fixed on a single card. It is rather interest- 

 ing, even seventy years after the original dis- 



covery, to record that this advantage can be 

 secured by a slight modification of the same 

 principle which Sir Charles Wheatstone had 

 so brilliantly demonstrated. It was, indeed, 

 in reading his Original account that the idea 

 occurred to me of arranging the two mirrors 

 in such a way that they would give proper 

 reflections of two halves of the ordinary 

 stereoscope card. The device will be easily 

 understood from the accompanying diagram. 

 In using this device the eyes are placed just 

 above the card which is turned with its back 

 to the observer (Fig. 1). The slight inclina- 

 tion of the mirrors brings it about that each 

 eye sees only one view of the card, while the 



li'iG. 1. The apparatus as seen from the side. 

 E, the eye; G, the stereoscopic card; M, the 

 mirror; H, the handle. 



combination is easily effected by a proper con- 

 vergence of the eyes to a common meeting 

 point beyond the plane of the mirrors. It is 

 an incidental feature of this device that it 

 dispenses with the necessity of the bridge or 

 screen which in an ordinary stereoscope is 

 necessary to prevent each eye from seeing 

 both views. This is unnecessary because the 

 image of the other view of the card falls out- 

 side of the field of vision of the one eye.* 

 There is no advantage to be maintained for 

 this form of the stereoscope; indeed, it has a 

 disadvantage which in certain cases is slight 



* This is practically the case ; yet with a full- 

 sized stereoscopic picture (8-314") there will be 

 a small portion of the outer edge of the left-eye 

 view visible to the right eye, and vice versa. 

 This is not seriously disturbing, and coiild be 

 eliminated by appropriate screens. 



