92 APOCHROMATIC OBJECTIVES 



dispersive power than the former. By this means the spherical 

 aberration is corrected for otie colour in the brightest part of the 

 spectrum, and the chromatic aberration is reduced by uniting two 

 of the colours of the spectrum in one point of the optic axis. 

 The remaining colours deviate and form what is known as the 

 * secondary spectrum.' The above-named corrections represent 

 the highest conditions of achromatism previously attained even in 

 the very best lenses. 



The new German crown and flint glasses, possessing different 

 relations of dispersing and refracting power, have, in combination 

 with a lens of fluor spar, made it practicable to destroy the secon- 

 dary spectrum, or very nearly so, by uniting three of the colours in 

 one focus, and at the same time to correct the spherical aberration 

 in two colours. 



This higher range of achromatism Prof. Abbe calls ' apochro- 

 matism.' It was a great advance in microscopic optics, and seems 

 to leave little room for further progress in the present state of 

 optical science. 



In wide-angled objectives, where, by reason of its shape, the 

 front lens cannot be made achromatic, there remains still a trifling 

 colour deviation which interferes with the perfection of the image. 

 This Prof. Abbe corrects by the construction of what he calls 

 ' compensating ' eyepieces or ' oculars,' in which the under-correc- 

 tion of the objective is balanced by the over-correction of the eye- 

 piece, and so a perfect image is obtained. And in order that these 

 eye-pieces may be used with all his objectives, Zeiss makes them 

 all under-corrected, so as to be balanced by the compensating 

 eye-pieces. 



Another advantage accrues to these eye-pieces by the new 

 method of their denomination. The old and almost meaningless 

 denomination by letters — (A, B, C, D, etc.) — is discontinued, and, 

 instead, they are designated by numbers — (4, 6, 8, 12, 18, and 27) — 

 each indicating the initial magnification of the ocular. By this 

 means the total magnification of the object viewed is most readily 

 ascertained. It is only necessary to multiply the initial magnifica- 

 tion of the objective by that of the eye-piece (as ascertained by its 

 number) to give the combined amplification. The power of the 

 objective is readily found by dividing the length of the microscope 



