46 CORRESPONDENCE. 



intended. Mr. Hogg having called notice to the article, I venture to 

 remark that chromatic and spherical aberrations are quite distinct in 

 their characters. It is well known to practical opticians that a tele- 

 scope lens can be corrected for colour, leaving nothing but the effects 

 of irrationality, and yet require all the curves to be remodelled, in 

 order to correct spherical aberration only ; and a telescope when so 

 corrected has the front and posterior surfaces of the object-glass of 

 different radii. If the lenses of the object-glass are reversed in the 

 cell, all fine definition becomes lost. 



According to Dr. Pigott's theory, so long as achromatism is ob- 

 tained ANY form of lens will suffice ; but we are far from realizing 

 this felicitous state of things. Photographic lenses must now be 

 made to work visually with perfect telescopic distinctness, in order 

 to meet the requirements of photographers. But as the chemical 

 plane of distinctness is within the visual focus, the lenses have to be 

 very much under-corrected, and the sharpness of picture is obtained 

 or the spherical aberration corrected, entirely by the curvatures given 

 to the combination. 



I am, Sir, yours obediently, 



F. H. Wenham. 



The Reflex Illuminator. 



To the Editor of the * Monthly Microscopical Journal.^ 



Sib, — As you have properly intimated that your pages must now 

 be closed against any further discussion with Mr. Stodder in relation 

 to his objectives, therefore beyond what I have already recorded I 

 shall make no remarks. 



But, as Mr. Stodder asks me a question in his second letter, I ask 

 permission to reply. When I stated, " water between the front lens," 

 I appear to have omitted the words and cover. All light being thrown 

 on top surface of slide, at an internal angle beyond that of total 

 reflexion, the field is quite dark with objectives of the largest aper- 

 ture — not the most extreme rays of which can admit them. All 

 transparent objects adherent to the total reflecting surface, cause the 

 rays to enter — without perhaps much deviation — and the object being 

 irregular in surface, or internal structure, the light is arrested and 

 dispersed, and the minute parts rendered visible with a dark field. 



If the object is mounted in Canada balsam, no such condition 

 exists, as the top of the cover then becomes the total reflecting surface. 

 If the objective is now used as an immersion, there will be no 

 reflexion from the cover, for the light now enters the object-glass. 

 But if the object is mounted dry, and lays on the slide, then the 

 object-glass may be used as an immersion, with a dark field, because 

 the cover in fact becomes nothing more than an additional thickness 

 of front lens, and acts optically as part of it. 



I am, Sir, yours obediently, 



F. H. Wenham. 



