ON THE USE OF THE MICROSCOPE. 589 



ought to be recognised. The other is the process of coagulation, which is also 

 produced by the influence of the surrounding fluid upon the substances under 

 observation. In this respect great caution ought to be used in the examination 

 of organic bodies, since apparent fonnations are frequently produced by such 

 coagulation, with which the nature of the thing has nothing to do. The best 

 rule in this case is always to examine organic objects in as fresh a state as pos- 

 sible, and to prefer unconditionally the image which exhibits itself at first sight, 

 to all others, and to regard it as normal, when a frequent repetition of the obser- 

 vation has convinced us that we have taken a correct view of it at the first sight. 

 Meyen has frequently described and represented coagulations of mucus and other 

 substances as forms (cells), for instance, in his Physiologic, III., Plate X., fig. 6. 

 Mirbel has done the same in his work, Sur le Cambium, &c., Plate XX., fig. 2. 

 Finally, we must direct particular attention to the second point above mentioned, 

 and which must be substituted for the exorbitant general requirement, viz., that 

 the microscopical observer should, before he proceeds to any research, make 

 himself most intimately acquainted with every thing that has already been 

 observed and made known with regard to the respective objects of his research. 



We now proceed (to make use of a medical expression) to the second indica- 

 tion, namely, to the many-sided comprehension of one and the same object. To 

 do this in the preparation of an object for microscopical examination, we must 

 consider how we may obtain from such object, properly prepared, as many views 

 as can possibly be obtained from it, in order to construct a clear image from the 

 combination of the individual conceptions. In this respect there is least diffi- 

 culty in the observation of opaque objects, since the object is here brought in any 

 manner we please into the focus of the object-glass, or of the simple lens. It is 

 simply laid in a suitable position on a small glass plate, and the latter on the 

 stage of the microscope ; or it is taken up between the small forceps, which are 

 usually given with every microscope, by which we obtain the advantage of turning 

 it round under the microscope, thus enabling us to view it on all sides. 



The observation of transparent objects is attended with much greater difficulty, 

 and,in fact, they form generally the objects of more accurate scientific examinations. 

 The object, in itself, is seldom so transparent as to enable us to bring it under the 

 microscope in an unprepared state. The wetting with water or with other liquids, 

 as fixed or volatile oils, Canadian balsam, &c., is of great assistance. Generally 

 we are compelled to make fine sections of the object, which, when sufficiently 

 thin, are always transparent, as, amongst organic substances, which form the most 

 important objects, we have no perfectly opaque objects. A double knife has been 

 invented for making such sections, which, however, is only calculated for a very 

 few botanical objects, its performance being anything but perfect. (Valentin, Re- 

 pert, vol. iv.). There is in fact no alternative but that of obtaining the neces- 

 sary skill by practice, so as to enable us to cut very fine slices by hand. The 

 anatomical scalpel was in former times commonly made use of for this purpose. 

 Subsequently very thin two-edged knives, in the shape of the grafting-knife, were 

 recommended, instead of the scalpel. I have found that a good razor with a 

 sufficiently heavy blade is the best instrument ; it may either be used merely with 

 one's hand, or by putting the object between one's thumb and fore-finger, and then 

 cutting through it. In this manner an accurate section may easily be obtained of 

 small objects, which may again be taken in the same way between the fingers, and 

 a thin slice cut off as 'before. If the object happens to be very delicate and 

 thin, as, for instance, hairs, the leaves of moss, &c., it must be attached to the 

 nail of the thumb, by means of a little oil or saliva ; the edge of the razor is then 

 placed upon it diagonally, and a to and fro motion is made with it, gently 

 advancing towards the root of the thumb. In this manner we may readily 

 obtain a number of thin segments, of which some are always perfectly available. 

 One great difficulty which we have to get over in this process consists in the 

 great softness of the object, which opposes so little resistance to a knife, that 

 even the sharpest blade tears and crushes rather than cuts it. I have invented 

 a method, which I have often applied with great success, in order to remove this 

 evil, and several of my friends have also availed themselves of it in the observ- 

 ation of animal substances. We first prepare, namely, a very concentrated solu- 

 tion of pure colourless gum arabic, and soften the object to be investigated 



