66 CLARKE, ON NERVE-FIBRE. 



cerebrum and cerebellum, and for the spinal cord of Rodentia, 

 Reptiles, and Fishes, the solution must be three or four times 

 weaker. Spirit is used to wet the knife in making the sec- 

 tions, which are placed in spirit for a few minutes, and then, 

 if thin, floated on the surface of spirit of turpentine. Here 

 they remain until they are quite or nearly transparent, when 

 they are removed to glass slides, on which a little Canada 

 balsam has been previously dropped. If now examined under 

 the microscope, they frequently show but little traces of cells 

 or fibres ; but if they be set aside for a time, and treated occa- 

 sionally with a little turpentine and Canada balsam, the cells 

 and fibres will reappear, and present a very beautiful ap- 

 pearance. Before they are finally covered with thin glass, 

 they should be examined at intervals under moderately high 

 powers. If the sections be thick, I find it best to place them 

 in a shallow vessel simply wet with turpentine, which can 

 therefore ascend from below, while the alcohol evaporates from 

 the upper surface ; for the principle of the method is this, — to 

 replace the spirit by turpentine, and this by Canada balsam, 

 without drying the sections. The method at first presents 

 some difficulties, and practice is necessary for complete suc- 

 cess; but when properly conducted, it affords many great 

 advantages, which no other method that I am acquainted 

 with can supply* Not that I wish to imply that this, unlike 

 every other method, is in every respect perfect ; nor do I 

 always confine myself exclusively to its use ; for whenever I 

 find that chromic acid, alone, offers greater advantages on 

 any particular point, I avail myself of it ; as I do of any 

 other means that appear most suitable for the occasion. 

 Sometimes I find it useful to colour the sections, according 

 to Gerlach's plan, before they undergo the process already 

 described, but I think it rather interferes with sharpness of 

 the fibres. The sections should be washed free from the 

 spirit employed in cutting them, and then immersed for a 

 few hours in an ammoniated solution of carmine of a deep 

 rose-colour, and previously filtered through paper; for, with- 

 out these precautions, a deposit or crust is apt to form on 

 the object, in consequence of the precipitation of the carmine ; 

 and for the same reason the section, on being removed from 

 the carmine, should be again washed in water before being 

 placed in spirit, and then in turpentine. 



Such were the carmine-coloured preparations which Messrs. 



* This met hod of rendering sections transparent lias been adopted in 

 Austria by Lenhosslk (with Borne slight modification); by Gerlach, in Ger- 

 many ; and quite recently, by Schrocder van der Kolk, in Holland. 



