ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 303 



decidedly very elegant, but the divisions of the drum of the micro- 

 meter-screw are so large and easily visible, even to weak eyes, as 

 in Prof. Thoma's opinion to make such complications useful only 

 for very special conditions. 



Other improvements by different manipulators relate merely to 

 secondary points, and do not touch the essential principles of 

 construction. 



Taking the hardened specimen directly between the arms of the 

 clamp is generally not advisable, as by such a proceeding sections of 

 great delicacy cannot be obtained. It should be fastened with gum 

 arable to the even surface of a square piece of cork, and the latter 

 inserted in the clamp. In this way compression is avoided. A con- 

 centrated solution of the gum is placed on the surface of the cork, 

 and the hardened specimen is watered a few moments to drive away 

 the alcohol from its surface, and it can then be adjusted on the 

 gummed cork and plunged again into alcohol. The latter will in a 

 few hours harden the specimen as well as the gum, and we obtain a 

 preparation like fig. 55. 



These methods are sufficient for the great majority of cases, and 

 the different animal and vegetable 

 tissues can be cut into sections varying Pig. 55. 



according to their structure between 



0-030 and 0*005 mm. Sometimes, J /;, 



however, and always if sections of 



extreme delicacy are required, it is 



necessary to use more complicated 



procedure. For example, the normal 



human lung hardened in alcohol and ^/" 



prepared as above, will perhaps admit i^. 



of sections of • 030 mm. ; a human Hardened specimen 6 adapted 



lung affected by acute pneumonia may to cork a. 



perhaps be cut to 0*015 mm., but if 



greater delicacy is required, the tissue must be soaked in gum arabic, 



or other substance which admits of a more solid hardening. In this 



case human lung will allow of sections down to 0*007 mm. Objects 



of very small dimensions, like embryos, small animals, leaves of 



plants, &c., must be imbedded in suitable masses, which may be 



adapted to a cork as above before they are cut. 



Imbedding Methods. — Prof. Thoma adds to his description of 

 his microtome some remarks on the imbedding methods more gene- 

 rally used. The method of treating tissues with gum arabic, first 

 brought into use by Eindfleisch and Eanvier, is now very generally 

 known and practised. The same may be said of the method of cutting 

 sections between two pieces of elder pith or hardened liver, &c. 

 These in certain conditions are very useful and simple, but other 

 methods of imbedding of more recent date give sections of the utmost 

 perfection and unsurpassed delicacy. 



The method of imbedding in emulsions containing fat and 

 albumen, originated with Bunge, and was subsequently modified by 

 Calberla and Euge. The following is very nearly the formula of the 



