Immersion Illuminators. By J. May all, jun. 31 



success approaching perfection in Tolles's " traverse-lens," which I 

 hope to place before you shortly with the inventor's notes. 



Many experimental devices have been made for the same pur- 

 pose. At the last meeting I exhibited another modification I had 

 had made of Dr. Woodward's " simple device " ; also a nearly hemi- 

 spherical lens and a small semi-cylinder conveniently adapted for 

 use on the sub-stage. 



I mention Hyde's oblique illuminator for its novelty in com- 

 bining a condenser with prism-illumination. It is a right-angled 

 prism with a lens of short focus cemented on the long face, and will 

 give a beam of condensed light up to a high degree of obliquity. 

 Caj)tain Tupman brought it from America four years ago. I am 

 not aware whether the inventor designed this for the purpose of 

 utilizing by direct transmission the ea'^ra-oblique rays that can be 

 utilized only by immersions having " interior angle " beyond 82^, 

 or he intended such rays to produce dark-ground illumination 

 only. The plan is ingenious. I have, however, found by cementing 

 a small lens on one of the exposed faces of Dr. Woodward's prism 

 the same results are obtained more conveniently. 



I refer also to a plan of illumination which Captain Tupman 

 informs me is due to Mr. ToUes. It consists of placing a suitable 

 prism in immersion- contact, on the surface of the balsamed slide, so 

 that rays from a bull's-eye lens may pass directly to the internal 

 surface of the base of the slide at an inclination beyond the critical 

 angle, they are then totally reflected to the object. This requires 

 some care in the manipulation. 



Professor Abbe has adopted the use of a small lens * placed in 

 immersion-contact with the base of the slide ; which is a very 

 simple and eflfective plan, and has been known for some years past. 

 It is really so practical as almost to supersede the more elaborate 

 contrivances for use beneath the stage. 



Lastly, I refer to a reflecting immersion illuminator which I 

 have suggested to Professor Abbe, and which he has undertaken to 

 have made for me by Mr. Zeiss : this w ill be placed before you 

 when completed. 



Immersion illuminators are designed to secure a particular an- 

 gular direction to the illuminating rays while actually in the body of 

 the fluid in which the object is immersed, with a view to utilizing 

 incident light of great obliquity ; used in connection with the highest- 

 aijgled immersion objectives, they have given fair grounds to expect 

 that the future of the most difficult investigations in microscopy will 

 be largely dependent on their successful aj^plication. 



* At the Meeting in June I erroneously stated that Mr. Wenham hail used a 

 siniilar lens for the same purpose many years ago. He used the leus for reflex 

 illumination from the cover-o;lass — « >/ for d rcct illumination. 



