MEDIUM-POWER PHOTO-MICROGRAPHY 77 



(v) The Movable Substage. — This stage is for the purpose of holding the 

 substage condenser. In most forms of microscopes it contains the means of producing 

 up and down movement as well as side to side in all azimuths. It is well seen in the 

 illustrations of microscopes already given. But in the Zeiss pattern the arrangement 

 is different. The condenser, Fig. 55, is itself contained in a self-centreing jacket 

 and the whole mass is dropped into a sleeve on the substage which is only capable of 

 being made to move up and down. This is not desirable, indeed the substage arrange- 

 ments of the Zeiss model are its weakest point, for it is obvious that without special 

 arrangement no other condenser can be substituted for the Zeiss pattern on account 

 of the fact that all English models are made of le7is pattern, much smaller than the 

 Continental, and are not sold in centreing sleeves at all. In our own case, to use the 

 Powell and Lealand condenser, we were obliged to have made by Beck & Co. a special 

 self- centreing arrangement to carry any condenser, as all the English ones are made with 

 the universal thread, the same in fact as the thread of the objectives themselves. 



It is a good addition for critical work to have a fine adjustment to the substage 

 to produce extremely delicate movements up and down. Several of the figures shown 

 have this, but it is absent in the Zeiss models, which is a subject of regret to the photo- 

 micrographer, and we are desirous of calling the attention of the firm to the subject. 



SECTION II.— (i) SELECTION OF OBJECTIVES 



Lenses are of two kinds, the achromatic and the apochromatic. The former have 

 been in existence for years and are being slowly and steadily improved both in quality 

 and definition, whilst the latter are of more recent invention. So far as the superiority 

 which the apochromatics possess over the achromatics, there is no possible room for 

 doubt, but seeing they are so very much more expensive and that achromatics of 

 recent years — ^since the introduction of the Jena glass — have reached to so far a 

 greater pitch of perfection than formerly, it will be advisable to discuss the use of 

 both kinds. 



The Achromatic is built on an entirely different principle to the apochromatic, 

 for even in the best makes, the inevitable residual colour of the secondary spectrum 

 is always well marked : indeed, with this type it is impossible with our existing 

 knowledge to entirely remove it. Opticians have striven for years to improve their 

 general performance, but achromatics have undergone a decided advance quite 



* It may interest the reader if he cares to go further into the subject, to read an article by the author in the "Annual 

 of Microscopy " for 1898, publii^hed by Percy Lund & Co., "Achromatics versus Apochromatics," 



