WITH THE MICROSCOPE. 1 95 



affirms the doctrine that living things may be produced by the 

 coalescence of separate lifeless particles, and increase and multiply 

 by the division of the resulting mass. It need, however, scarcely 

 be stated that facts now known render such a notion untenable. See 

 a controversy upon this subject in the British Medical Journal, 

 January, February, March, 1864. 



On the Accidental Presence of Extraneous Matters. Cleanliness is 

 of the utmost importance in every branch of microscopical enquiry 

 and without great care many substances of extraneous origin may be 

 introduced into the specimen about to be examined, and the observer 

 may mistake the character of the objects introduced accidentally for 

 those of the special objects under examination. Particles of starch 

 or other solid bodies may gain entrance into a tissue submitted to 

 examination, and the observer may be led to conclude that these 

 bodies were embedded in the substance of the texture. 



When we consider how minute many of the structures rendered 

 evident to the eye by the microscope are, we shall scarcely wonder 

 that many light substances are liable to come in contact with the 

 specimen which is under examination. The cotton or flax fibres 

 from the cloth, starch globules which adhere to the thin glass (for 

 the small pieces are often kept in starch), portions of feathers, 

 various kinds of hair and oil globules are among the substances 

 which are most frequently met with in examining different structures, 

 and I need hardly say that their presence is purely accidental. 

 That I am not giving needless caution upon this head, is shown 

 by the fact that in a well-known and highly valuable publica- 

 tion, a drawing of what is evidently a portion of feather is described 

 as a representation of lymphatic vessels, vegetable hairs are 

 described as nerve fibres, and several other errors equally unpardon- 

 able occur. Now, such mistakes could only arise from utter 

 ignorance of the characters of some of the commonest objects with 

 which every observer ought to be very familiar. I would very 

 strongly recommend every one to study the characters of all these 

 substances before he attempts to make any original observations. 

 He is sure to meet with them from time to time, and the sooner he 

 is well acquainted with their characters the better. 



The following should be very carefully examined : 



Oil globules, milk, pi. XIX, figs. 124, 126. 



Potato, wheat, and rice, starch; and bread crumbs, pi. XXXV II I, 



figs. 241, 241*, and pi. XLIV, fig. 2757,-. 

 Portions of feather; worsted, pi. XLIV, fig. 275. 



o 2 



