26 The Microscope. 



To Dr. J. E. Talmage, Salt Lake City, for beautifully finished 

 photo-micrographs of Artemia fertilis from Great Salt Lake, 

 and of the head of Taenia crassiGolia from liver of brown rat. 



NEV/S • FRp/A- 

 THE V^ORKERS 



Note on semt-apochromatic objectives.* — Of late 3'^ears optical 

 progress in the making of microscope objectives has been great, 

 but all is not 3'et accomplished, and Dr. S. Czapski, of Jena, in 

 a recent article, has stated with much clearness what it is still 

 desirable to do. 



Zeiss's apochron'atic objectives mark a considerable advance 

 toward perfection. The}^ have the advantage of being well cor- 

 rected for the spherical aberration due to those rays which have 

 the o-reatest chemical action, as well as for those which make an 

 impression on our retina. It therefore happens that to the eye 

 the visual field of the mlcrosco[)e presents more light, whilst 

 the distinctness of the image is very great ; still, as M. Czapski 

 has well remarked, these apochromatics have in realit}^ but 

 a slight advantage over homogeneous immersion objectives. 

 But for the making of photographic negatives the advantage is, 

 on the contrar3', considerable, and experience will more and 

 more conclusively demonstrate the great superiority of the 

 apochromatics for photo-micrography. 



But these lenses have three objectionable features: 1st. 

 Their price is very high. 2d. The glasses used sometimes 

 undergo changes. These are the phosphates and the borates 

 which, under diverse atmospheric influences, and especially 

 undei the slow action of water (even when vaporised), tend 

 toward crvstallization. These 2;lasses then become more or less 

 opaque and worthless. We should state that Zeiss has always 

 exchanged, free of expense, these altered objectives, and that 

 the recent glasses furnished b}' the Schott works only rarely 



*Prof. J. Bran. — Joarnal de Micrographie. 



