Angular Apertures of Ohjed-glasses. By Jabez Hogg. 207 



screen, a cube of glass greyed on the under surface should be so 

 placed tbat the lens may be accm-ately adjusted and fociissed on 

 the upper surface in water conlact, or still better, in glycerine. 

 The angle can then be read off the luminous disk on the greyed 

 surface, by applying a suitable tangent scale. I have some idea 

 that Mr. Wenham first proposed a cube of glass for measuring 

 apertures, but have been unable to verify this, although I feel sure 

 that the lenses so measured were dry lenses only. No true aperture 

 can be measured unless the lens is so adjusted as to give its best 

 and finest definition ; it is therefore utterly fallacious to attempt to 

 measure the aperture of an immersion lens unless it is adjusted for 

 and actually measured in immersion contact. It would be quite as 

 fallacious to attempt to measure the aperture of a dry lens unless 

 adjusted for and measured as a dry lens. If the lens is so 

 contrived that it can be used either wet or dry, then the aperture 

 will necessarily be greater when used wet than dry. I possess a 

 Dallmeyer's ^th, which can, by suitable adjustment, be used either 

 wet or dry. The angle when used wet is 70^ measured in the 

 cube of glass, and 55° when accurately adjusted and used dry. 



It may give increased weight to what I have said to add that 

 the modification of Professor Robinson's method spoken of has been 

 submitted to Professor Stokes by Mr. J. Mayall, jun., and he 

 admits its validity ; and furthermore, Mr. Mayall has employed it 

 in my presence for measuring the aperture of various immersion 

 lenses ; amongst others, Tolles' \\h, belonging to Mr. Crisp, and 

 the measurements so made confirm Mr. Tolles', that is, the 

 aperture of the lens in question was found to be nearly 100^ in 

 the cube of glass. It is but right I should state that the aperture 

 of the same lens was measured by Mr. Wenham in my presence, 

 by his semi-cylinder and turn-table method, and with a metal stop 

 of -so of an inch in diameter fixed in the focal plane, and the 

 measurement very nearly approached 100°. 



I shall not enter upon the question of the utility of slits or 

 metal stops for measuring immersion apertures, as I believe Pro- 

 fessor Keith's criticism in the Journal for December last* effec- 

 tually disposes of this part of the subject. I will, however, direct 

 attention to the fact that Mr. Ingpen appears to think the slit is 

 properly used when " separated to the exact diameter of any field 

 of view, thus excluding all but ' image-forming rays.' " Those 

 microscopists who have followed Mr. Wenham's more recent state- 

 ments as to the right use of the slit will at once perceive that 

 whilst he insists the slit should admit only the merest line of light, 

 Mr. Ingpen thinks it should not encroach on the field of view. It 

 is to be presumed that Mr. Ingpen has given the subject due con- 

 sideration, and I hope he will at some future time favour the 



* Pflge 284. 



VOL. XV. U 



