80 PHOTO-MICROGRAPHY 



The Apochromatic System, of which we shall now speak, is necessarily a very 

 intricate one, for a very large number of lenses enter into the formation of one 

 combination. They are therefore necessarily very costly, but the resulting effect is a 

 perfectly colourless image of great excellence, brilliancy, and purity. Readers are 

 warned to be exceedingly cautious in having anything to do with apochromatics 

 which have been brought out of cheap design, claiming to be as good as the more 

 expensive ones. We have examined most carefully one of this make, and found that 

 it was not perfectly apochromatic, as stated, and that its N.A. was but slightly oi?er 

 1*26, although sold as i"40. 



Photographically speaking, one great advantage of the real apochromatic system 

 is that photographs can be taken in any colour, for all rays of the spectrum are 

 sensibly brought to a common focus. It will be seen at once how very important this 

 is, especially when deahng with objects of feeble contrast in colour, such as the 

 bacteria of diseases. It may be mentioned that owing to their feeble staining, in some 

 instances the only chance of making a good photograph is to use some coloured screen, 

 whereby a colour contrast can be produced between a bacillus and its background. 

 This done, a good photograph can be obtained provided the lens focusses the same in 

 all colours, but it is equally obvious that if an ordinary achromatic were used the 

 final result must of necessity — there is no discussing the matter — be greatly inferior. 

 We have only to look at the photographs taken of these objects in past years by 

 achromatics of the then highest order, and compare them with similar objects taken 

 in the present day by means of apochromatics ; the result speaks for itself 



The sura total, then, of the remarks on lenses comes to this : — ^When photo- 

 micrographs are desired of the very finest order and of high amplification, such as 

 photographs of diatoms or bacteria at 1000 or 2000 diameters, nothing will give the 

 perfection required save the apochromatic used ivith its compensating eye-2nece; if, 

 however, medium power work up to, say, 200 or 300 diameters, is the sole ambition 

 of the photographer, then he may confidently rest assured his results will be worth 

 looking at should he employ achromatics well corrected for photography. 



It is an open question whether photographically corrected achromats should be used 

 with an eye-piece or not. Personally we have no hesitation in saying so ; but it is 

 equally well known that in seme photographers' hands with certain specimens, and 

 certain degrees of magnification, excellent results have been obtained without. But a 

 doubt still remains in one's mind even then whether better results still would not have 

 been produced should an eye-piece have been used. With the apochromatic system, 

 inasmuch as part of the final correction of the objective is obtained by the use of the 



