The Microscope. 27 



undergo the change which has been called the apochromatic 

 disease 3d. The third inconvenience iiiherent in these lenses 

 is in the la3'er of fluor-spar there used. This soft, exfoliating 

 material makes them delicate and fragile. 



M. Albert Brun, chemist, Licentiate of Science, has this year 

 [1891] succeeded in artiticiall}- producing a substance having 

 almost exactly the properties of flour-spar, but harder and 

 more resisting. It is opal, a rare gem, found in Nature only in 

 small pieces, and, in a word, too dear to be used practically. 

 (See Archives des science physiques et natarelles^ Geneva, June, 

 1891.)"^ I read in that article tliat "the house of Carl Zeiss has 

 acquired the right to make and to use this artificial opal for 

 optical purposes." 



We should also speak of the progress of a very desirable 

 consummation, that of making the objectives at a lower price. 

 I have this 3ear had occasion to visit the workshops of M. Kor- 

 istka, an optician at Milan, and I have admired the precision 

 with which his instruments are made. His mountings, his iris 

 diaphragms, his micrometer screws, etc., leave nothing to be 

 desired so far as mechanical perfection is concerned. This 

 optician has appreciated the advantanges of the apochromatics 

 for direct observation with the eye, setting aside the optical 

 combination useful for photo-micrograph}-. With this object 

 he makes a 1-15-inch homogenous immersion objective which he 

 calls " Semi-apo,^' and wliich he sells at the moderate price of 

 200 francs [$40.00J, including two compensating oculars. 



This objective is Aery advantageous to those that do no photo- 

 micrographic work. The illumination is admirable and the 

 definition exceedingly sharp. Slightly oblique light resolves 

 the striae of Navicula crassivervis {Frustulia saxonica), and 

 with axial illumination the fine striae of Surirella gemma are 

 resolved into pearls. The objective is well adapted to natural 



*M. Albert Brun has sneceeded iu making artificial opal, in transparent, 

 colorless, homogeneous and amorphous masses. He has determined the 

 following optical constants. ^ ^^ ^ ^ These values are in accord with 

 what Prof. Abbe has found for different kinds of natural opal. ' ' We 

 remark that among vitreous substances the artificial opal is one of those whose 

 index is the smallest. The process of manufacture produces pieces large 

 euougli to be conveniently used in the construction of optical instruments. 

 The house of Carl Zeiss, at Jena, has accj[uired the right to manulacture and 

 to use this artificial opal for optical purposes. ' ' 



