30 Transactions of the Society/. 



properly speaking, unless the image of a point be rendered as, ap- 

 proximately at least, a point. 



Now, although, as I have shown above, Mr. Wenham understood 

 the need of special means for illuminating obliquely objects in 

 balsam, and the importance of the angle of illumination in relation 

 to the aperture of the dry objective, I do not think he can be 

 credited with having understood (much less foreseen) the important 

 part the immersion illumination of balsamed objects would take in 

 the development of the fullest power of immersion apertures. 

 Indeed, as he has contended that the 82° Uiuit of dry objectives 

 obtains equally in immersions, he must be held to deny the existence 

 of any aperture beyond S'Z° : consequently, the application of the 

 immersion illuminators above mentioned, for directly utilizing any 

 such aperture, must be regarded as a discovery quite apart from his 

 original application of them for dark-ground illumination. 



It appears to me that to Mr. Tolles is due the merit of first ap- 

 plying immersion illuminators to balsamed objects in connection 

 with immersion objectives for the distinct purpose of utilizing by 

 direct transmission the excess of " interior angle " beyond 82^. He 

 was the first to produce objectives having interior angle considerably 

 beyond 82^, and to demonstrate their advantages. With these ob- 

 jectives a luminous field was obtained when the whole of the illu- 

 minating rays that can enter into a dry objective were blocked out, 

 and none but rays beyond this limit admitted : thus exhibiting at 

 once a luminous field and a definition of immersed objects by means 

 of the extra aperture that had not been seen before. He appears 

 to have experimented chiefly with the semi-cylinder, because of the 

 facility it offered for immediately obtaining a reading of the precise 

 degree of inclination the illuminating rays made with the axis, so 

 as to determine the actual limit of the apertures of the objectives 

 he had devised ; the display of difficult test-objects being merely 

 incidental to his efforts to improve the instrument. 



Dr. Woodward has given special prominence to the principle of 

 the immersion illumination, in its immediate connection with the 

 development of the power of aperture, by his " simple device," in 

 which he originally provided means to exclude all rays of less incli- 

 nation in glass than 45*^ from the axis, so that no objective having 

 "interior angle" less than 9 O'' would give a luminous field with it : 

 it thus aflbtds a proof of his position in the aperture question. 

 Viewing it as an illuminator only, Dr. Woodward has simplified the 

 mode of mounting the prism, and slightly varied the angle in a 

 second prism : his last paper referred to these changes. I was 

 also led to design a modification of this device, which is, briefly, to 

 utilize the four exposed surfaces of the prism by cutting them at 

 different angles so as to approximate nearly to the semi-aperture of 

 the objectives likely to be used. This purpose is attained with a 



