330 THE AMERICAN MONTHLY [Dec, 



I am not aware that this combuiatioii of changed focus and 

 streoscopic effect has ever before been suggested or practiced. 

 Certaiidy, if it has, like the practice of stere()SC()|)ic photomicro- 

 graphy, it has not rece ved th^ attention which its value demands. 



It is not necessary here to enter into the theory of stereos(!opic 

 vision for jiside from the theory of stereoscopy it is known that 

 in ordinary 8tereosco|)ic photogr.iphy full relief and sharpness 

 is obtained in the combined imige even if one of the pictures 

 is blurred or indistinct. The shj^rp outlines of tlie 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 impression 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 througli a stereo- 

 scope the sharp oudines 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. For instance, if an attempt is made to photogra])h a di- 

 atom like the Actlnoptychus unduhUus, 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 photomicrographs must be taken of such an object 

 in ordt-r to properly picture it; and if these pictures are taken 

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

 all parts appear 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 toscinodiscus valve. The right 

 hand picture was taken with ihe surface of ihi^. Heliopelta awd 

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

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

 ings of the same objects. Inspection of these pictures will 

 show that the. sectors of the Heliopelta and parts of the Coscin- 

 odiscus fragment which are sharp in one p cture are blurred in 

 the other; yet when viewed through the stereoscope all are sharp, 



