ZOOLOGY AND BOTANY, MICROSCOPY, ETC, 617 



Prof. Zenger, in view of Blair's experiments, determined to see 

 whether it would not be possible to find fluids which in combination 

 with crown glass, would produce achromatic objectives. The con- 

 ditions for absolute achromatism require that the partial dispersions 

 should maintain the same relation in all parts of the spectrum for the 

 two refracting media. Now mixtures of aromatic and fatty substances 

 possess this property to a high degree of approximation, so that when 

 combined as lenses with crown glass (a biconvex crown and a plano- 

 concave fluid) all the different rays of the spectrum will be united 

 and a perfection of achromatism will be produced not hitherto 

 attained. 



The question of course arises whether the fluids in consequence 

 of striae-formations, through rapid changes of temperature, may not 

 originate a new element of optical imperfection. This is opposed to 

 the author's experience of fluid achromatics in sunlight, either with 

 the telescope or Microscope. He has succeeded in converting the 

 ethereal and fatty oils which serve for the production of the refract- 

 ing media, into the condition of vitreous bodies, or into a kind of 

 gelatine in which strige-formation is not as easily possible as in very 

 mobile fluids. 



By solutions of stearic, oleic, or palmitic acid, or mixtures of 

 these, we can change benzol, castor-oil, poppy-oil and other similar 

 ethereal and fatty oils into transparent gelatine, which is amorphous 

 like glass, perfectly clear and does not flow out of the vessel if in- 

 verted. These substances are already used in the arts. 



An immense scope for combination is thus opened in order, so to 

 say, to produce kinds of glass of any desired refraction and dispersion, 

 and consequently the optician is saved the trouble of undertaking 

 changes of radius at great expense and loss of time. It is sufficient 

 to make a suitable selection of the gelatine substance which is to 

 be inclosed between a plane parallel plate and the biconvex lens, in 

 order to solve the hitherto difl&cult problem of a perfect achromatic 

 and aplanatic lens-combination. 



The closing up of the fluid must be as hermetical as possible, in 

 order to prevent any evaporation and chemical change in the course 

 of time. There are ethereal and fatty oils which are transparent and 

 very little changeable. 



The problems as to lenses for telescopes. Microscopes, and photo- 

 graphic objectives are therefore, it is claimed, extraordinarily 

 simplified through the use of " endomersion " objectives, which are 

 thus named by the author in analogy with immersion objectives, 

 because the fluid is between the lenses. On account of the fact that 

 three radii are equal, while the fourth is infinitely great, he also 

 calls them " symmetrical " endomersion objectives, a quality which 

 embraces the most favourable conditions for brightness, sharpness, 

 and flatness of field of view. 



Formulae and tables are given for the construction of endomersion 

 objectives, and after considering more particularly the case of teles- 

 cope objectives, those for the Microscope are dealt with, in which case 

 the plane side of the concave fluid lens should be turned to the object. 



Ser. 2.— Vol. IV. 2 T 



