6 The Microscope. 



and place in oil of cloves; allow it to remain here until the tissue 

 around the eggs assumes a transparent, glassy appearance. Then 

 remove to a thin balsam solution and inount at pleasure. Turpeiltine 

 does not seem to answer for clearing purposes, as it made all the 

 specimens that I immersed in it, quite opaque. I have mounted a 

 number of specimens, prepared as above described, in " Hard Finish," 

 recommended by Mr. Seaman at the recent meeting of the A. S. M., 

 and it appears to answer the purpose as well as balsam, is pleasanter 

 to handle, and easier to prepare. It may be obtained of any dealer 

 in painters' supplies, and is prepared by being thinned with benzole, 

 and then filtered. If perfectly free from small particles of dust, 

 when bought, even this latter precaution is unnecessary. 

 High School, Pittsburgh, Pa. 



APOCHROMATIC OBJECTIVES.* 



ERNST GUNDLACH. 



I HE almost generally prevailing opinion, that the microscope 

 -*■ objective has been brought so near to perfection as to leave little 

 or nothing for its farther improvement, has been greatly modified by 

 the appearance of a new and superior material of which to construct 

 optical lenses — the apochromatic glass of Schott & Co., of Jena, 

 Germany. The fact that this new glass has solved the long- pending 

 problem of removing or reducing the secondaiy spectrum, has natur- 

 ally aroused the most sanguine hopes for a general improvement of 

 the microscope objective. These hopes would doubtless long ago 

 have been realized, through the efforts of the able opticians of the 

 world, if the new glass did not have, aside from the great virtue of 

 reducing the secondary spectrum to a minimum, some serious draw- 

 backs not connected with other optical glass. In fact, if the new 

 glass were, or could be made, in every respect similar to the ordinary 

 optical glass, the objectives could be made of it in exactly the same 

 manner and after the same formulae as they are now, and their 

 optical qualities would be just the same in every respect, but, with 

 the secondary spectrum considerably reduced, and, consequently, the 

 definition greatly improved. But, unfortunately, this is not the case. 

 In my paper, at last year's meeting, I pointed out the fact, derived 

 from figures of the refractive and dispersive powers of the new glass, 

 as furnished by the makers, that the proportions of powers were such 

 as to require extremely short curvatures, which would produce a very 



*Read before the American Society of Microscopists, Pittsburgh, Aug, 30, 1887. 



