M. Mohl on the Growth of Cell- Membrane. 147 



dots are so transparent, and appear so bright in the coloured cell- 

 membranes, especially when they have acquired a deep indigo 

 tint, that by an illusion they look like true openings. But to 

 make the fact certain, we must ascertain accurately the perform- 

 ance of our microscope, and carefully select the suitable objective 

 and covering glass of the proper thickness, regulate the proper 

 illumination, in short, we must neglect no circumstance which 

 may influence an important microscopical examination. Since 

 the question, whether in these young cells actual openings are 

 present or not, is one of the principal hinges on which the doc- 

 trine of the development of cell-membrane turns, I may be per- 

 mitted to enter somewhat minutely into the qualifications of the 

 microscope employed by me in these investigations. I am in- 

 deed, generally speaking, of opinion that the accuracy of a mi- 

 croscopical observation does not depend upon the fact of the 

 microscope being a little better or worse, since experience in ob- 

 servation frequently counterbalances the inferiority of the in- 

 strument ; but I consider that the present case is one of those 

 in which an instrument of the most superior quality is necessary, 

 and in which we cannot come at the truth without a microscope 

 of great penetrating power. I commonly make use, in import- 

 ant investigations, of the three strongest of Plossl's objectives 

 (N. 5-7), with an Amici's achromatic ocular, since this combi- 

 nation gives an image of surpassing sharpness and clearness with 

 a magnifying power of about 300. Notwithstanding the supe- 

 rior performance* of this combination, I was not in a condition 

 ever to detect any trace of a membrane closing the dot in the 

 young medulla-cells of Sambucus, since the light shone through 

 perfectly bright and clear, and apparently quite uncoloured as 

 through a true opening. But when I used the strongest of 

 Amicus objectives, which can only be employed with profit in 

 few cases and to very delicate and transparent objects, and which 

 with the same ocular gives a magnifying power of 500 diameters, 

 every doubt was dissipated as to whether a membrane was 

 stretched over the dot or not, since such a membrane was now 

 distinctly to be perceived : it was indeed very transparent, but 

 small granules, &c. were distinctly to be seen adhering to it. If 

 this was not to be mistaken in the preparation which was lying in 

 water, the result of the examination of dried preparations was 

 yet more decisive, since no doubt was longer possible as to the 

 presence of a closing membrane, and of the bright violet colour- 

 ing of the same. 



* For instance, the transverse stripes on the scales of the upper side of 

 the wing of Hipparchia Janira are quite clearly seen with it ; these scales 

 afford an object which cannot be sufficiently recommended for testing the 

 microscope. 



M2 



