282 CHAPTER XX. 



physical point of view, and frequently afford a better defini- 

 tion of delicate detail than Canada balsam does. But I 

 very strongly suspect that no da-mar solution is perfectly 

 stable. A review of some old damar mounts has shown that 

 the majority of them have developed granules that have 

 deteriorated the preparations to a greater or less extent. 

 (These granules are in the worst cases large enough to at 

 once attract attention even with low powers ; at other times 

 they are so small that they can only be seen with the highest 

 powers, and in this case may be mistaken for normal elements 

 of cells.) Xylol-balsam and benzol-balsam mounts are in 

 the same case, but to a less degree. Chloroform balsam 

 keeps much better, so far as granules are concerned. But 

 it becomes very brown with a#e, and has the defect that it 

 is injurious to stains made with coal-tar colours. Seiler's 

 alcohol-balsam keeps remarkably well, but it also will not 

 preserve the coal-tar stains. For these and other stains I 

 now often use turpentine colophonium. It gives very good 

 definition of delicate detail, and keeps perfectly. (Dr. PAUL 

 MAYEK, however, writes me that turpentine solutions are not 

 at all good for alum-haematein stains.) Turpentine colo- 

 phonium has a rather low index of refraction for objects 

 that require much clearing. For these I very frequently 

 use oil of cedar wood in preference to any resinous medium. 

 It keeps perfectly. With time it thickens sufficiently to 

 hold the cover in place ; or, if desired, preparations may be 

 luted with BELL'S cement. After using an oil immersion 

 objective 011 a fresh mount, it is always easy to change the 

 cover by floating it up with a drop of the oil placed at the 

 side. 



Another reason for preferring turpentine colophonium, 

 where possible, is that it does not shrink in drying nearly so 

 much as the media made with volatile solvents. 



Still another motive is that turpentine media preserve the 

 index of visibility of the preparations much longer than do- 

 media made with volatile menstrua. Preparations made with 

 these last become so transparent in course of time that much 

 fine detail is often lost. (Such mounts may, however, be 

 revivified without removing the cover by putting them for a 

 day or two into a tube of benzol ; the benzol penetrates the 

 balsam, and brings it down to a lower refractive index.) 



