CORRESPONDENCE. 285 



ing the objective in a glass tank containing the medium. It seems 

 impossible for Mr. Tolles to comprehend this. With the tank 

 nothing can go wrong. No focussing is required, and whatever the 

 depth of immersion, the ray passes straight through the bottom, and 

 fluids of any refractive power may be used with equal accuracy, the 

 results remaining correct with all lengths of focus clue to the refrac- 

 tion of the different fluids. Though this is certain, Mr. Tolles may 

 undertake some unfathomable optical demonstration to disprove this, 

 which may be amusing, but will need no answer. 



Immersed apertures may of course be taken first, as employed on a 

 balsam object, and the increase of aperture when dry, afterwards 

 measured by the usual method. Fallacies have at times appeared of 

 aperture measurements, with the lenses closed within the distance of 

 definition. Mere light can be seen through a hemispherical lens up 

 to 180°, far beyond the perception of distinct images. 



Finally, I may assure Mr. Tolles that the inflexible laws of light 

 will not bend to meet his wishes, and as he puts forward, without the 

 pale of disinterested argument, some extravagant advantages in his 

 own " peculiar objectives " when used as immersion, which he is 

 " almost certain that no English objectives will be found to have," 

 therefore, for my own part, I willingly accept the challenge, being 

 rather sure that object-glasses made in this country intended to act 

 as immersion, will keep their aperture, measure for measure, under 

 similar conditions to anything that he can produce. 



I have no doubt that the committee with any one else that 

 Mr. Tolles might name, would act again — if possible, to settle this 

 aperture question. Of course nothing need be stated concerning the 

 relative performance of object-glasses unless he desires it. 



F. H. Wenham. 



The Angular Bange of Objectives. 



To the Editor of the 'Monthly Microscojrical Journal.' 



Boston, April 17, 1873. 



Sir, — I desire to put on record the fact that I have, using the semi- 

 cylindrical appliance under the stage of my microscope, somewhat, in 

 the manner indicated in the sketch sent you last month, put through and 

 utilized successfully an angular pencil of 112° into cylindrical surface 

 through its substance of glass, through balsam, slide, balsam-mounted 

 object, glass cover, water, objective, infallibly to the eye, and of that 

 angular dimension, at the extreme of oblique incidence giving symme- 

 trical view of the object (N. Amicii), good definition and resolution. 



The objective was one of three systems, having a cemented triple 

 front. This is really my first attempt to exceed 100° of " image- 

 forming rays" from (through) object in balsam, with only three sepa- 

 rate systems. I hope very easily to exceed that angular range with 

 the aid of the cylindrical condenser, — of which I am not prepared at 

 present to give a more detailed description. 



By putting the light of lamp-flame down the open microscopic tube 



