STRUCTURE.] ARE PITS PORES? MICROSCOPES. 17 



there is the further advantage, that the preparation coloured 

 by iodine may be allowed to dry again, by which means, as 

 is known, the detection of a very thin and transparent mem- 

 brane is especially facilitated, I treated in this way the 

 medulla-cells of the young developing bud of Sambucus nigra, 

 Asclepias syriaca, and of the apex of the stem of Euphorbia 

 Caput medusae. The result of the microscopical examination 

 of these does not at all agree with that offered by Harting and 

 Mulder. It is certainly quite true that dots are so transparent, 

 and appear so bright in the coloured cell-membranes, espe- 

 cially 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 performance of 

 our microscope, and carefully select a suitable objective, and 

 a covering glass of proper thickness must 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 doctrine of the development of cell-membrane 

 turns, I may be permitted to enter somewhat minutely into 

 the qualifications of the microscope employed by me in these 

 investigations. I am, indeed, generally speaking, of opinion 

 that the accuracy of a microscopical observation does not 

 depend upon the fact of the microscope being a little better 

 or worse, since experience in observation frequently counter- 

 balances the inferiority of the instrument ; 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 important 

 investigations, of the three strongest of PlossFs objectives, 

 (No. 5 7), with an Amices achromatic ocular, since this 

 combination gives an image of surpassing sharpness and 

 clearness, with a magnifying power of about 300. Notwith- 

 standing the superior performance of this combination, I was 

 never in a condition 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, 

 VOL. i. c 



