ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 773 



more or less light which may be both refracted and diffracted by the 

 object. If the latter has a fine structure, periodically arranged, this 

 light reaching the outer zone of the objective's aperture will be mostly 

 of diffraction origin, and will take the form of separated spectra. As 

 the iris of the condenser is opened, however, it can be seen that the 

 diffracted beams expand to the same extent as the dioptric beam, finally 

 overlapping it and each other, until when the aperture of the objective 

 is entirely filled with dioptric light it must unquestionably be similarly 

 filled with diffracted rays. Unfortunately, there seems no way in which 

 the beams derived from the two different sources can be completely 

 separated, and the best expedient the author could devise to obtain some 

 idea of the conditions present was to insert a semicircular diaphragm in 

 the condenser, so oriented that the left side of the back of objective was 

 completely filled with light up to its margin, the right-hand side receiv- 

 ing no direct rays whatever. Thus there would be full-cone conditions 

 on one side, while the other would receive only refracted and diffracted 

 rays ; or, if certain suitable objects are employed, nothing but diffracted 

 rays whose behaviour could be separately studied. For this purpose a 

 binocular Microscope should be employed with a specially short-mounted 

 objective, say a sixth of about ■ 80 N.A., the back lens of which will 

 come close to the Wenham prism. All the direct light from the fully- 

 illuminated left half of the objective will now pass up the right-hand 

 tube of the Microscope, while the diffracted beams from the right-hand 

 half of objective will be reflected up the left tube. Assuming that 

 Phurosigma angulatum is the object, and oriented longitudinally across 

 the field in a right and left direction, on examination of the back of the 

 objective, the previously dark space on the right will be found practically 

 filled by three of the characteristic spectra of the object, and the other 

 three will be present, although invisible, in the illuminated half. Thus, 

 through the right-hand tube of the binocular, the image will be pro- 

 duced by a full dioptric beam supplemented by diffracted beams corre- 

 sponding to those resulting from a small central illuminating cone ; 

 while through the left-hand tube it is derived from diffracted beams 

 alone corresponding to those present with a full cone of illumination. 

 On examination it will be found that both images are fairly well defined, 

 but that the resolution of the fine structure is noticeably sharper and 

 more distinct in the diffraction image through the left tube. The fact 

 that the diffraction image is blue in colour is a proof that it is due 

 practically exclusively to diffracted rays. Still further proof is, however, 

 required to demonstrate that it is free from refracted rays. For this 

 purpose the author inserts a narrow slit diaphragm in the condenser, 

 thereby producing a sharp spectrum which can be magnified by a low- 

 power objective inserted in the draw-tube. The Microscope can now be 

 used as a spectroscope, and by allowing the light to pass through some 

 colouring matter (e.g. eosin) which has well-marked absorption bands, 

 these bands will be visible in the spectra at the back of the objective. 

 It will be found that the absorption band in the spectrum derived from 

 a diatom structure is perfectly black, thus furnishing a proof that it is 

 practically free from refracted rays, for if it contained scattered refracted 

 rays, as has been claimed, the absorption band would not appear black, 



Dec. loth, 1909 3 F 



