424 NATURE AND MAN. 



whilst the field over which our vision ranges with sufficient distinct- 

 ness to see our special object in combination with its surroundings, 

 is far larger than is attainable in any optical instrument of human 

 contrivance. 



I venture to think, moreover, that my special experience as a 

 microscopist has given me the means of adding something to 

 Professor Helmholtz's demonstration of the practical efficiency of 

 the eye. 



Until recently, it has not been found possible by the most 

 skilful constructors of the microscope to produce object-glasses of 

 high power and wide angular aperture, which should be perfectly 

 free from both spherical and chromatic aberration. This, however, 

 has recently been accomplished by what is called the "oil-immersion" 

 system ; but the correction can only be perfectly made for a certain 

 relative position of the conjugate foci ; — that is, when the object is 

 at the precise distance in front of the lens, and its image is formed 

 at the precise distance behind it, for which it is adjusted by the 

 maker. Hence, the principal continental constructor of these 

 lenses, Zeiss, of Jena, makes two forms of each power : one for 

 the short 8-inch body of the microscopes generally used on the 

 Continent, and one for the long jo-inch English body. Neither 

 of such object-glasses will work perfectly with a microscope of the 

 other length. For, in order that its image may be projected at 

 ten inches' distance, the object must be brought nearer to the 

 objective than when its image is formed at eight inches' distance ; 

 and this diminution will sensibly disturb the performance, on the 

 English microscope, of the combination which was perfectly cor- 

 rected for the Continental microscope ; whilst a disturbance in the 

 opposite direction will be produced by the increase of distance 

 between the object and the objective, which becomes necessary 

 when an objective corrected for the long English body is used 

 with a short Continental microscope. These disturbances will 

 alike affect the chromatic and the spherical aberration ; and 

 there is no known method by which they can be prevented. 

 In fact, I believe I may say that it is demonstrable that no 

 combination could be constructed, which should give perfectly 

 aplanatic and achromatic images at different focal distances. 



Mark, now, the superiority of the eye. In its normal condition, 



