588 APPENDIX. 



exception of the central rays) a refraction during their transit from the denser 

 medium into the air, and this diverts them considerably from the rays of the axis; 

 they strike, therefore, the periphery of the atmospheric globule, and on emerging 

 from it again experience a refraction, by which they diverge so far from the rays of 

 the axis, that they cannot arrive at the object-glass, and, consequently, not at the 

 eye. A similar process takes place with all gases enclosed in a fluid. This, even 

 at the present day, is a stumbling-block. We meet with elaborate explanations of 

 a dark material said to be deposited in the membranous glands, and also with 

 theories based upon the observation of them ; but, on strict examination, we find 

 that it is only the air enclosed in the stomates which has deluded the observer. 

 Now we have plenty of means of convincing ourselves whether we have air before 

 us or not ; for instance, water, which soon imbibes the air, caustic potash, alcohol, 

 oil of turpentine, &c. ; but we ought to expect from a practised observer that he 

 should be able to distinguish air from a solid substance by merely looking at it. 

 Air contained in the intercellular passages has also been described as a dark juice. 

 On the other hand, air has been sought for where it can never be found. In man) 

 works it is still stated that " the epidermal cells contain air." It only requires 

 a glance through the microscope, and some elementary knowledge of optical 

 science, to convince us that nothing more than fluid, which has nearly equally 

 refractive power with water, is contained in the epidermal cells of any healthy 

 living plant. But such matters are committed to paper, and copied again, without 

 any one inquiring about their correctness or asking for reasons. 



Drops of oil present the same appearance under the microscope, only with this 

 difference, that the black margin in the oil- drops is much narrower, owing to the 

 difference of refractive power between air and water being greater than between 

 oil and water, and a greater number of rays are therefore lost in the air-bubble. 

 The explanation is the same in this case as was given when speaking of the air, 

 only the rays take exactly the opposite directions, owing to the greater refractive 

 power of the oil. 



Other thick fluid substances, for instance mucus (protoplasma), assume 

 different forms in fluids with which they are mixed, and in which they are not 

 dissolved, and which forms are generally caused by their adhesion to other 

 objects, as, for instance, to the surface on which they are examined, and in that 

 case they are fibrous or membranous ; if, on the other hand, they are more iso- 

 lated, and left to their own cohesive power, they assume a spherical form. 



B. There are also processes very generally met with which a microscopic 

 observer ought to be acquainted with, in order not to be deceived by them when 

 they occur. Certain motions, first of all, belong to such. 



a. Robert Brown, the gifted English botanist, first made the important dis- 

 covery, that all substances, organic as well as inorganic, on being suspended in a 

 fluid in sufficiently small particles, are in a state of constant trembling or vibratory 

 motion, similar to a mass of monades, when seen with a low magnifying power. 

 The motion is difficult to be characterised, and it can only be accurately compre- 

 hended, and distinguished from other similar motions, by frequent observation. 

 It has been frequently observed in parts of plants, for instance, in the fine 

 granular contents of the pollen-cells, and has been described as a special living 

 action, which it certainly is not. We know nothing yet of the origin of these 

 movements ; but they are probably owing to slight electrical tensions and com- 

 pensations. 



b. Another movement, which is frequently seen, is produced when two different 

 fluids, which have a considerable affinity for each other, for instance, water and 

 alcohol, or water and solution of iodine, are mixed with one another. A 

 powerful current usually takes place in them, frequently in quite opposite 

 directions. 



c. A third case is, when fluids rapidly evaporate. During this process there 

 usually takes place a double current, namely, an upper one, from the margin to 

 the centre of the drop, and a lower one, from the centre towards the margin. 



d. There are two further occurrences to be observed, which give rise to fre- 

 quent illusions ; one of them is the process of solution. Since most objects are 

 in a fluid, when under observation it frequently happens that a solution of many 

 objects takes place. The movements and changes of form occasioned thereby 



