Objectives 55 



is to say, the secondary spectrum was practically removed, 

 and spherical aberration was very perfectly corrected for 

 all colours. The objective, therefore, produced, to all 

 intents and purposes, a colourless image. Higher aper- 

 tures were obtainable, and in consequence of the improved ■ 

 corrections, accompanied by greater brilliance of the field, 

 the use of eyepieces of high power was rendered permissible 

 and advantageous. 



The new kinds of glass were placed at the disposal of 

 opticians throughout the world, and apochromatic objectives 

 have been since manufactured by other firms, and notably 

 by Messrs. Powell and Lealand, of London, whose produc- 

 tions compare favourably with the best of the originators' 

 lenses. The apochromatic objectives by Zeiss have their 

 equivalent focus engraved in millimetres, and it is becoming 

 usual for the same method to be applied to other objectives 

 also. The initial magnifying power of such lenses is ascer- 

 tained by dividing the equivalent focus in millimetres into 

 250. Thus, a lens with an equivalent focus of 2"5 milli- 

 metres would have an initial magnifying power of 100 

 diameters. 



Special eyepieces, termed ' compensating oculars,' are 

 necessary when using the apochromatic objectives. They 

 will be found described on page 79. 



Achromatic Objectives. — All objectives that are not 

 actually comprised in the apochromatic category — that 

 is, in which the secondary spectrum is not eliminated — 

 are included under this heading. So far as the principal 

 opticians are concerned, it comprehends a better class of 

 objectives than it did at the period when apochromatic 

 lenses were introduced. By the use of the new optical 

 glasses previously referred to, and in consequence of keen 

 competition amongst manufacturers, many achromatic ob- 

 jectives, tending towards apochromatism, have been made. 

 Several of these are so well corrected that in some instances 

 they vie with the apochromatics in performance. 



The class has consequently arisen which has been 



