366 OBJECTIVES, EYE-PIECES, THE APERTOMETER 



lations of a most elaborate and exhaustive kind made by Dr. Abbe, 

 objectives are made by Zeiss which not only combine three parts of 

 the spectrum instead of two, as formerly, but are also aplanatic 

 for two colours instead of for one. This higher stage of achromatism 

 Abbe has called apochromatiim. 



A general plan of the cor. struction of an apochromatic objective 

 as made by Zeiss is shown in fig. 319, which, it will be understood, is 

 diagrammatic, but sufficiently illustrates the elaborate corrections by 

 which the perfect results given by these objectives are accomplished. 

 But, in addition to their form of construction and the special optical 

 glass of which they are composed, it is now known that they owe 

 much of their high quality to the use of fluorite lenses amongst the 

 combination. Fluorite is a mineral which has lower refractive and 

 dispersive indices than any glass that has yet been composed, and 

 therefore by its introduction the optician can reduce the spherical 

 and chromatic aberrations greatly below that reached by achromatic 

 combinations of the known type. 



It is a somewhat depressing fact that fluorite is very difficult to 

 procure in the clear condition needful for the optician, but from what 

 we have seen the optician can do in the manufacture of glass, we 

 may hope that an equivalent of this mineral in all optical qualities 

 may be discovered. 



The medium for mounting and immersion contact has, of course, 

 to be of a corresponding refractive and dispersive index in all ob- 

 jectives of great aperture, and it is insisted by Abbe that the glass of 

 which the mount is made, both slip and cover, must, when the limit of 

 refraction by crown glass is passed by the objective, be of flint glass. 

 This he presents as a sine qua non in the case of the new objective 

 made a few years since by the house of Zeiss, and a specimen of 

 which has been generously given by the firm to the Royal Micro- 

 scopical Society. This glass has a numerical aperture of 1'63; in 

 a subsequent chapter on the present state of our knowledge as to 

 the ultimate structure of diatoms we are enabled to present the 

 results of some of the photo-micrographs produced by its means. 

 But it may be noted that very much will depend upon the N.A. of 

 the illuminating cone which can be employed with it not theoreti- 

 cally, but practically, and it is for practical purposes of no value to 

 the student of minute life, because the highly refractive and dis- 

 persive medium needed to make the object mounted homogeneous is 

 destructive of life, and even of organic tissues. Such value as it 

 may have is therefore confined entirely to the examination of 

 silicious and other indestructible organic or inorganic products. 



Before leaving this part of our subject we note with pleasure 

 that Mr. Nelson has computed a triplex front of minimum aberra- 

 tion suitable for an oil-immersion condenser. We illustrate it 

 in fig. 320. The data for this are as follows, viz. : 



O is the object and V its virtual image ; the hyperhemispherical 

 front is aplanatic for these two points. The scale of the drawing is 

 arranged so that the distance of the vertex A of the front lens to 

 the object is one inch. The three lenses are made of borosilicate 

 glass, No. 5 in the Jena catalogue, ju = l'51 ; and as the reciprocal of 



