HIGH-POWER OE CRITICAL PHOTO-MICROGRAPHY 



141 



of obtaining the "perfected image by finding the suitable position for the auxiliary lens, 

 and also the advantage of using Mr. Conrady's perfectly achromatised i '35 condenser, for 

 we reduced the time so considerably that my son actually obtained a first-rate negative 

 with only thirty seconds' exposure, although using the F line screen. The closeness 

 of the lines differs with various specimens. In our case we measured them and found 

 they were about 100,000 to the inch. Hence, if an ordinary hair of the head was split 

 into about 480 strips, one strip would about fill the space between any two ! 



To photograph these lines as clots is the highest record for the photo-micro- 

 graphical student. According to some great authorities it can only be done with 

 a lens which has an aperture of over i'40, which means an expense of no small 

 amount, seeing that a special condenser is also required, let alone a slip and cover- 

 glass cut and polished from a block of glass having the actual refractive index 

 required for the optical combination.^ 



Dr. Van Heurck, however, it is believed, somewhere about 1884, with some 

 special form of silvered preparation, made a photograph showing the lines resolved into 

 beads, but in 1887 actually succeeded in photographing them by transparency, em- 

 ploying monochromatic solar illumination. We have never seen the photograph, but 

 in 1889 the same renowned microscopist and photographer took what must be con- 

 sidered as his opus magnus, a photograph of them at 2000 diameters with the new 

 lens. Here we must pause, for we know many authorities entir'ely disagree with the 

 reality of these pictures, believing them to be nothing but photographs of optical 

 phenomena, for the reader must be reminded we are dealing with objects now that 

 are becoming commensurate with the wave lengths of light. We admit ourselves to 

 be very uncertain, for it is a known fact that the phenomena displayed by the Pleuro- 

 sigma angulatum have been the subject of mathematical inquiry, and it is maintained 

 by no less an authority than that of Professor Abbe himself, that the mathematician 

 can accurately show what the details of this image should be in direct accordance with 

 the number of diffraction spectra that the objective itself has transmitted. Hence, in 

 the case of this diatom, theory indicated the optical but not necessarily the structural 

 existence of the tiny markings. Dr. Eichhorn actually made the calculations of their 

 size and shape without knowing of their existence, and when Mr. Stephenson re- 

 examined the object with annular light he actually saw them, and Dr. Zeiss has since 

 been able to photograph them. If, then, this be true with the Pleurosigma angu- 

 latum, why should it not apply to the Amphipleura pellucida equally well ? 



* The obliquity of the light to show the lines should be at right angles to them — up and down the specimen ; but in 

 their direction, that is, across the specimen, to show the dots. Monochromatic sunlight must be used for the latter. 



