268 CORRESPONDENCE. 



losing by common reflexion, while the other loses more by total re- 

 flexion ; but the angles themselves remain equal." 



Fixing our attention on these two pencils, let us see where this 

 theorem leads us. He has admitted that " more of the pencil of light 

 is lost in the air lens than in the other from total reflexion," which dis- 

 poses of pencil No. 2 — the dry lens pencil. And, now, applying his 

 theorem we learn that more of the pencil of light is lost in the immersion 

 lens from common reflexion ! which disposes of pencil No. 1 — the 

 immersion lens pencil — in a passing strange manner, not admitting of 

 proof by diagram. 



Moreover, in the ' M. M. J.,' No. xxii., p. 238, Mr. Brakey says " the 

 immersion lens would lose somewhat less light than the other by 

 ordinary reflexions." 



I do not mean to suggest that Mr. Brakey cannot explain his 

 paradoxes ; but till he does so in an intelligible [form, I shall, with 

 sorrow, be obliged to hold to my " not flattering " opinion of his 

 writings. 



Your obedient servant, 

 EusTicus, jun. 



Mr. Tolles and Dr. Koyston-Pigott. 



To the Editor of the ^Monthly Microscopical Journal.^ 



Ilkley, neab Leeds, May 14, 1874. 



Deae Sir, — I have received a letter enclosed by Mr. Stodder from 

 Mr. Tolles, in which he expresses his opinion " that most readers will 

 confound angle aperture with field aperture." 



" Why not write Dr. Pigott, calling his attention to the difference 

 between reduced aperture and reduced field?" 



He goes on to say, quoting my expression — " Bad telescopes cured 

 of residuary aberration by contracting the aperture" never define well 

 anywhere with the full aperture (this everybody knows well enough). 

 " But the ^-inch objective defines just as well with the full aperture as 

 under any degree of contraction : only the maximum (the best) effect 

 is gained through but a limited extent of field." 



" Dr. Pigott must know about this distinct difference between 

 angle aperture and field aperture, while most readers will confound 

 the two as he does in expression. If you will call the Doctor's 

 attention to the matter, no doubt he will make correction." 



To these remarks I hasten to say that I am sorry Mr. Tolles 

 should think it necessary to write about this matter, as I suppose 

 very few persons whose opinions are worth anything would make so 

 egregious a blunder as to imagine using an eye-piece with a very 

 small field of view contracted the angular aperture of the objective. 

 The point of my remark in the foot-note, page 175 of last number of 

 the Journal, is substantiated by Mr. Tolles' own admission, namely, 

 that, as he says, " only the maximum effect is gained through but a 

 limited extent of field." 



Indeed, I stated in effect " that in a limited field of view the de- 

 finition of Tolles' |th is superb : beyond the central area the definition 



