48 correspondence. 



The Slit as an Aid in Measuring Aperture. 



To the Editor if the ' Monthly Microscopical Journal,^ 



Sir, — I do not gather from Professor Keith's communication that 

 in the measurement of aperture he brings forward any argument against 

 the use of the slit for preventing false light frome ntering the object- 

 glass. This light has hitherto deceived many. Let any objective be 

 adjusted to uncovered, so that all aberrations at this point are cor- 

 rected : the outer plane of the slit is then set in the focus, in the way 

 that I have described. The slit need not be very narrow, but is 

 opened out to the full extent of the field of view : it is now absohitely 

 correct for uncovered aperture, from which it cannot cut off any rays 

 coming to the focus. It may be argued, that if the object-glass is now 

 closed for " covered " and tried again with the slit in the most distinct 

 approximate focus, that by cutting off the aberrant rays of the pencil, 

 as showTi in Professor Keith's diagram. Fig. 2, it should show less 

 aperture, but such in reality is not the case ; the aperture has increased 

 in the usual ratio from uncovered to covered. But of course I do not 

 maintain that this is correct. If immersion apertures are to be taken 

 with the front lens and slit immersed, the object-glass must be adjusted 

 for immersion and the focus brought to the plane of slit — and this way 

 I have always used it. The slit does cut off" false light seen up to near 

 the impossible angle of 180^, and in no object-glass that I have yet tried 

 by any maker, d(jes the immersed angle exceed 82°, but in all cases has 

 fallen far short of it. 



As false positions have been drawn by opponents of ways to use 

 the slit improperly, in order to convey the impression that after having 

 devised the instrument I do not know how to use it rightly, I can 

 see no use in arguing the question. I am always ready to prove my 

 case by giving a demonstration ; but in America I cannot do this for 

 some time to come. 



I am. Sir, yours obediently, 



F. H. Wenham. 



Hasert's Objectives. 



To the Editor of the ' Monthly M icroscoj/ical Journal.^ 



Sir, — I have good reason for believing that even Mr. Hickie's 

 guarded statements with respect to Hasert's objectives will not bo 

 endorsed by those who are better acquainted with their perform- 

 ance ; no one will be inclined to accept the vague and uncertain 

 utterance put forth, that they " are capable of perfoi*ming truly 

 wondrous feats :" indeed, it would appear that no very exalted opinion 

 is entertained of Hasert's lenses by his own countrymen. 



Dr. Dii.pcl, in liis work ' Das Miki-oskop,' p. 168, 1872, tells us 

 " that tlie beauty and clearness, definition, of images when viewed by 

 direct illumination with Hasert's objectives, cannot be for a moment 

 compared with the performances of either Hartnack or Amici's im- 

 mersions. They are found wanting in the resolution of lined objects 



