ON THE USE OF THE MICROSCOPE. 575 



Page 97., add at line 3. 



We have recently received an excellent treatise on the origin of these 

 motions, from H. v. Mohl (On the Motion of the Sap in the Interior of 

 the Cell. Botanische Zeitung, 184-6, p. 73.). He demonstrates how 

 several cavities filled with watery fluid are gradually formed in the young 

 cell, originally filled with homogeneous protoplasma ; how these cavities 

 expand, by degrees meet, and thus at last displace the protoplasma to such 

 an extent that it only forms a thin layer on the internal surface of the 

 cell, with thickened places in it, filaments as it were, some of which run 

 across the cell ; while the motion simultaneously commences in all these 

 filaments, or perhaps now first becomes visible through granules begin- 

 ning to appear in the originally homogeneous protoplasma. I can wholly 

 confirm this account. 



Page 104., add to line 17. 



Hugo Mohl, in a revision of his first treatise*, has placed the matter 

 beyond all doubt. See Appendix, page 572, line 30. 



D. 



ON THE USE OF THE MICROSCOPE IN BOTANICAL 

 INVESTIGATIONS. 



CONSIDERING the experience of the last thirty years, there is no need to observe 

 that a profound study of any of the natural sciences, and especially of organisa- 

 tion, is impossible without the aid of the microscope. He who expects to become 

 a botanist or a zoologist without using the microscope, is, to say the least of him, 

 as great a fool as he who wishes to study the heavens without a telescope. I 

 will therefore say no more respecting the value of this instrument. But as 

 yet there has been no satisfactory work on its use, owing to the deficiency of a 

 proper theory of vision itself. I will therefore attempt to give some indications 

 in this respect. 



The conception of distance is the result of a mathematical judgment. We 

 must consider accurately the conditions connected with the simplest cases. We 

 take up images on the retina at first as luminous points, and afterwards as 

 planes, situated beyond us. The lines on the different points of this surface 

 form angles among themselves, and these angles, in various directions, are next 

 apprehended. But it is evident that these angles differ according to the different 

 distances of these various points from the eye. All relative determinations of size 

 must, therefore, first be mathematically constructed, the starting-point of which is 

 evidently the size of the angle of vision. The second element would be the dis- 

 tance, of which also we become gradually conscious through comparison of many 

 impressions, the angle of vision being again the simple foundation for this ; since 

 we generally place things at a greater distance when they appear to us under 



* On the Multiplication of Vegetable Cells by Division. Verm. Schrift., 1845, p. 362. 



