A New Freezing Microtome. 185 



test Poclura, the molecules of cancer cells and other diseased forms, 

 monads (minute atoms of metal by reflected light might also be 

 named), is at present impossible ; when such delicate forms are in 

 quest, all rays of an aberrating character must necessarily be 

 extinguished. 



B. — Eye-^ieees. 



The spherical aberration of positive eye-pieces may be examined 

 as follows : — 



The positive in an inverted position is placed under the stage, 

 and forms an image of the solar disk for examination. — Proceedings 

 of the Rotjal Society, Vol. XXI., No. 146, June, 1873. 



[The fifth Plate belonging to this paper, representing the 

 spectra exhibited by a globule of mercury, is at j)resent omitted, but 

 it may possibly reappear in a future article.] 



III.— .4 New Freezing Microtome. By William Piuthekford 

 M.D., Professor of Physiology, King's College, London. 



All microscopists have experienced the difiiculty which attends 

 the cutting of soft tissues into thin slices. Valentin's knife is 

 continually had recourse to for this purpose; but although by 

 means of it excellent sections may be made of an organ ha\dng a 

 density such as that of the kidney, it is by no means so successful 

 in the case of mucous membrane, lung, or even in that of the 

 skin ; and for the brain, spinal cord, and the dehcate tissues of 

 the eye, spleen, thyroid and lymphatic glands it is useless. Such 

 tissues require to be hardened ere they can be successfully shced. 

 They are usually hardened by means of chromic acid, potassium 

 bichromate, Miiller's fluid, alcohol, &c. In general, the hardening 

 process requires considerable time, varying from several days to 

 several weeks; and, after all, the tissues undergo alterations 

 which, although advantageous in the case of some, are not so in 

 the case of others. The vitreous humour is quite destroyed, 

 lymph spaces are very often obhterated, soft tissues are sometimes 

 shrivelled. The method of hardening the tissues by freezing has 

 long been recognized as one of great importance in enabhng the 

 histologist to obtain sections of fresh tissues, but it has hitherto 

 been little adopted owing to the inconvenience and clumsiness of the 

 methods proposed. 



In May, 1871, I pubhshed in the ' Journal of Anatomy and 

 Physiology' an account of a microtome invented by me for the 

 purpose of facilitating the process of freezing and of cutting frozen 

 tissues. The apparatus there described and figured, although 



