INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 885 



adopted. Tho front lens is greater than a hemisphere, and the 

 surface is active in the production of the image up to several degrees 

 beyond the equator, so that the lens is mounted on a thin glass plate, 

 and the slightly prominent edge of the latter fixed to the brass work of 

 the objective. 



" F. R. M. S.," writing on this subject, says : * — " This plan of 

 mounting front lenses on a thin plate of glass so that the setting need 

 not encroach on the active spherical surface, seems to have been known 

 to and practised by the late Andrew Eoss in connection with dry 

 lenses. Some ten years ago ToUes, of Boston, experimented with this 

 plan of mounting, for water-immersion lenses. But I believe it is 

 not on record that either Ross or Tolles ever attempted to utilize 

 a front lens beyond the hemisphere. 



The first notice I have met with, relating to the possible use of 

 a front lens greater than a hemisj^here, is in a paper ' On the 

 Question of a Theoretical Limit to the Apertures of Microscopic 

 Objectives,' | by Professor G. G. Stokes, of Cambridge. Professor 

 Stokes there discussed the question from a theoretical jioint of view, 

 and gave a demonstration, based on the assumption that such a frout 

 lens could be utilized, proving the possibility of ajiertures approxi- 

 mating to 180^, measured in the body of the lens. 



The first practical development of this idea — whether suggested by 

 Professor Stokes's paper or not, I am unable to say — was projected by 

 Professor E. Abbe, of Jena University, and successfully applied by 

 Zeiss, the optician, of Jena, under Professor Abbe's direction, to 

 extend tho apertures of homogeneous-immersion objectives to the 

 highest point hitherto attained, 1*4 numerical ap. (= 131°, nearly, 

 measured in crown glass of mean index 1 • 525). 



In June 1879, Professor Abbe brought over to England one of 

 these high-angled J objectives. He exi)lained at the R. M. S. that he 

 had found it necessary to prepare a special immersion fluid (an 

 aqueous solution of chloride of zinc) for use with the new lens, 

 because he had not found it possible to obtain satisfactory correction 

 of the aberrations with any of the refractive fluids previously in use. 

 Even with the zinc solution he found it important to improve the 

 corrections by a novel chromatic refracting device of his own con- 

 trivance, to be placed immediately below the eye-piece. While this 

 immersion medium remained in the desired condition, tho definition 

 obtained with the lens was remarkably good ; but, unfortunately, the 

 solution quickly became turbid and useless, so that Professor Abbe 

 did not venture to exhibit tho lens at work in public. Ho stated that 

 the difficulties of construction would probably preclude Mr. Zeiss 

 from making such lenses for sale. I Lad the good fortune to see tho 

 lens tested under the most favourable conditions, and can aflirm that 

 it produced excellent results. 



In certain demonstrations conducted by Professor Abbo he jiointcd 

 out the fact that tho now lens could bo utilized for proving tho 

 refractive indices of various immersion fluids ; for example, using 



* 'EiikI. Mc'ch.,' xxxi. (18S0) j>. 517. 

 t See this Jouiiml, i. (\mn) p. I'SJ). 



