1886.] 



MICROSCOPICAL JOURNAL. 



231 



slide should be readily dehydrated ; 

 hence it is sometimes necessary to 

 wash them again with fresh 96 per 

 cent, alcohol as they lie upon the 

 slide. The sections are now well 

 covered with a pei^fectly clear 10 to 

 12 per cent, solution of refined shel- 

 lac, and the slide at once exposed to 

 a gentle warmth (3o°-40° C. ) until the 

 shellac is completely dried. It is pos- 

 sible to mount the preparations at once 

 in fluid balsam, but I find it more con- 

 venient, on account of the greater cer- 

 tainty of escaping air bubbles, to clear 

 up the shellaced specimen with oil of 

 cloves, (other essential oils may be 

 used as vv^ell). The oil may be re- 

 moved in the ordinary manner, by 

 cigat'ette paper, all the more readily 

 because the preparation will stand 

 pretty rough handling. The oil of 

 cloves ought not to be left on the 

 slide for more than half an hour, as 

 it gradually softens the shellac. 



The shellac used should be that 

 known as 'refined,' which gives a 

 perfectly clear solution. The sec- 

 tions should be thoroughly covered, 

 and the thicker they are the more shel- 

 lac is required. If sufiicient shellac is 

 used, the finest histological details are 

 preserved, so far as I have been able 

 to observe, unaltered during the dry- 

 ing process. Without enough shellac, 

 I'uinous shrinkage occurs. If the sec- 

 tions were imperfectly dehydrated 

 there appear opaque whitish spots in 

 them after the drying. In this case 

 wash the slide over with 96 per cent, 

 alcohol until the opaque spots have 

 disappeared ; drain oft" the alcohol by 

 tilting the slide, add a little fresh shel- 

 lac and dry again ; if necessary, repeat 

 the process until the dried specimen 

 has no trace of cloudiness. 



This shellac method has other ap- 

 plications. It can, of course, be ap- 

 plied to ordinary sections from which 

 the imbedding substance has been re- 

 moved. It is, however, particularly 

 serviceable for mounting teased prep- 

 arations, isolated cells, and small or- 

 ganisms. These may be transferred 

 from alcohol to shellac, and after they 



have been arranged in the latter on the 

 slide in such positions as mgy be de- 

 sired, the preparation is dried, and the 

 mounting may then be accomplished 

 without the fragments moving from 

 their place. 



Appochromatic Objectives and 

 Compensating? and Projection 

 Eye-Pieces. 



FROM DR. C. ZEISS' CATALOGUE.* 



In the construction of the appo- 

 chromatic objectives new glass and 

 greatly improved methods of correc- 

 tion have been employed, so that 

 there is a far more perfect concentra- 

 tion of the rays than heretofore, 

 and, in the case of the chemically ef- 

 fective rays, there is neither focal dif- 

 ference nor spherical aberration. 



They also allow very high eye- 

 pieces to be used without detriment 

 to the accuracy or brightness of the 

 image, thus giving high magnifying 

 power with relatively long focal 

 length, and enabling a series of am- 

 plifications to be obtained with the 

 same objective. 



The natural colors of objects, even 

 in the more delicate tints, are repro- 

 duced unaltered by these objectives, 

 in consequence of the very slight in- 

 tensity of the residual tertiary spec- 

 trum. The spherical aberration out- 

 side the axis is so completely corrected 

 that the sharpness of outline existing 

 in the centre of the field of view is 

 maintained almost up to the margin, 

 although the focal adjustment between 

 the centre and margins is necessarily 

 somewhat different in consequence of 

 the unavoidable curvature of the sur- 

 face of the image. • 



The construction of each objective 

 is based on calculations which extend 

 to the smallest details of optical ac- 

 tion. Every element, radii of ciu'va- 

 ture, thickness, diameter, and distance 

 of lenses from one another, are all ac- 

 curately adjusted and numerically de- 

 termined for each objective, with re- 



* Copied from article in Journ. R. Micr. Soc, 1886 

 p. 849. 



