CORRESPONDENCE. 265 



objects it has a decided advantage over the hirge-angled patterns, even 

 when their corrections are more perfect than its own. 



Zeiss makes three immersion glasses, Nos. 1, 2, 3, with focal 

 lengths in millimetres 3, 1 • 7, and 1 respectively ; one millimetre 

 being 0*039 of an English inch. He states that the highest of these 

 " may be used very well with a covering glass of above a fifth of a 

 millimetre in thickness." The power of this glass is stated to range 

 from 860 to 2400, according to the eye-piece used. The angles of 

 the three immersion objectives are stated at 180" ; but the following 

 remark is made in the catalogue, and may be taken as a contribution 

 to some of the discussions now going on. 



" The indication contained in the Table touching the angle of aper- 

 ture of the immersion systems does not treat the matter exhaustively, 

 inasmuch as in the case before us the amoimt of the angle cannot be 

 at all stated with accuracy in the ordinary way, that is to say, for air 

 as the external medium. The real angle of aperture of the above im- 

 mersion systems lies between 104° and 108° for water, whereas an 

 angle of only 97° for water would give an angle of 180^ in air. 



" That when compared with this great angle of aperture in the im- 

 mersion systems, the dry systems, even in the highest powers, should 

 have an angle of only 105°, that is to say, a materially less angle than 

 the high systems of numerous other makers, especially the English 

 opticians, is to be thus accounted for, namely, in consequence of theory 

 and practice agreeing in assigning 105° as the limit that may not be 

 exceeded in dry systems without either rendering the entire correc- 

 tion of the spherical aberration impossible, or reducing the distance of 

 the object so considerably that the systems become in consequence 

 extremely troublesome to use." 



The catalogue states that the whole of the object-glasses " are con- 

 structed in conformity with the theoretical calculations of Professor 

 Abbe, of Jena." Are these knou-n to our opticians ? and do they 

 differ from the formulae they employ, either in mathematical accm'acy 

 or in facilities for cheap construction ? This may be a point worth 

 looking to. 



. Yours, &c., 



Henry J. Slack. 



Mr. Wenham's Criticism of Dr. Woodward's Figure. 



To tJie Editor of the '■Monthly Microscopical Journal.^ 



Georgetown, D.C, May 6, 1874. 

 Sir, — I think Mr. Wenham's criticism of Dr. Woodward's figiire* 

 likely to mislead, for the reason that a well-known fact is advanced as 

 if inconsistent with the relative positions of the points F, E', &c., in 

 that figure. It is true that when a glass cover is placed over an 

 object in focus, the objective has to be drawn away and the distance 

 slightly increased. But it is also true that the object is at the same 

 time apparently brought towards the objective, and becomes an apparent 

 * ' M. M. J.,' No. Ixiv., p. 170. 



