330 THE AMERICAN MONTHLY [Dec, 



I am not jiwaro that this Combination of changed focus and 

 streoscopic elFect has ever before been suj^gested or practiced. 

 Certainly, if it has, Hkethe practice of stereoscopic j)hotomicro- 

 graphy, it has not received the attention wliich its value demands. 



It is not necessary here to enter into the theory of stereoscopic 

 vision ft>r aside from the theory of stereoscopy it is known that 

 in ordinary stereoscopic photoj;raphy full relief and sharpness 

 is obtained in the combined image even if one of the i>ictures 

 is blurred or indistinct. The sharp outlines of the good 

 picture will override the somewhat blurred outlines of the 

 poorer one while the combination of the two produces the im- 

 pression of relief. This being the case, it would seem probal)le 

 if a sharp impression of one plane of an object was obtained for 

 one picture and a sharp im|)ression of another plane for a sec- 

 ond picture, provided the planes were not so widely separated 

 but that the details of each still appeared, though blurred, in the 

 other; that when these pictures were viewed through a stereo- 

 scope the sharp oulines of each would override the blurred 

 outlines of the other and a s ngle stereoscopic picture would be 

 produced, presenting different planes of ihe object with sharp- 

 ness and with true relief. In practice this is found to be the 

 case. F'or instance, if an attempt is made to photograph a di- 

 atom like the Aclinoplychus unduldtus, it will be found that ow- 

 ing to the undulations of the diatom valve, no correct image of 

 the entire diatom can be obtained at any single focus; for ow- 

 ing to the undulations of the valve, the parts which are sharp 

 at the superficial focus are blurred at the deep focus, and vice 

 versa. Two j)hotomicrographs must be taken of such an object 

 in orcbr to properly picture it; and if these pictures are taken 

 and viewed stereoscopically, a true image of the object in which 

 all parts aj»pear sharp and in relief will be obtained. The illus- 

 tration for this article is from such a stereoscopic photomicro- 

 graph of a Heliopelta and broken Loscinodiacus valve. The right 

 hand picture was taken with ihe surface of th*^ Hdiopeltn and 

 the edge of the broken Coscinodiscus in focus while the left hand 

 picture was tjiken with the focus adjusted to the deeper mark- 

 ings of ihe same objects. Inspection of these pictures will 

 show ihat the sectors of the Heliopellfi and parts of the Coscin- 

 odiscun fragment which are .'•harp in one p cture are blurred in 

 the oiher; yet when viewe Ithrougfi the stereoscope all are sharp. 



