32G SUMMARY OF CURRENT RESEARCHES RELATING TO 



Ward's Iris Illuminator.* — R. H. "Ward, in order to obtain, with 

 oblique illuminatiou, the advantages obtained with the iris dia- 

 phragm with axial illumination, has devised the arrangement shown 

 in fig. 78. It consists of any desired lens-system, either dry or 



Fig. 78. 



Siimii ^iiiillllllliU^^^ 



immersion, under and close to which is mounted an iris diaphragm 

 ■with a decentering adjustment ; the diaphragm being set in a sliding 

 plate pushed by a screw or lever, so that it can be moved into any 

 position from the centre to the periphery of the system without alter- 

 ing the position of the latter. Thus not only tlie obliquity of the 

 light, but the exact amount desired or found advantageous at any 

 chosen obliquity, can be regulated with perfect precision by a touch 

 of the hand to the screw and to the adjusting collar of the dia- 

 phragm. 



A blue glass disk is fitted to the bottom of the dark well of the 

 diaphragm. A special adapter is also provided for the use, in place 

 of the iris, of central stops for securing dark-field illumination ; or 

 of a horizontal slit or pair of horizontally arranged apertures, for 

 the better illuminating of binocular Microscopes, as proposed by the 

 writer in the 'American Naturalist' for December 1870; or of any 

 special stops desired by the user; or of a polarizing prism and 

 selenite plate. The whole apparatus rotates about its own optical 

 axis, which remains coincident with that of the Microscope itself. 



It is used to the best advantage with a 4/10 achromatic con- 

 denser, or with the thick non-achromatic immersion lenses of the 

 Abbe condenser. It cannot, without outgrowing the limits of the 

 standard 1^ in. substage ring, be applied to the largest lenses now 

 used as condensers, and for this reason, if for no other, it might be 

 unavailable for extreme resolution with objectives of excessive aper- 

 ture. 



By removing the lens from the top of the apparatus, the iris 

 diaphragm, with or without its blue glass disk or the polarizing 

 prism, will be found in position for use by itself. 



* Proc. Amcr. Soc. Micr., 7th Ann. Meeting, 1884, pp. 160-1 (1 fig.). 



