ON THE USE OF THE MICROSCOPE. 591 



b. Mechanical Means. 



It is advantageous in many respects to observe how an object alters on 

 the application of pressure. A double disc was formerly made use of for this 

 purpose : this, however, was attended with the disadvantage that we could only 

 observe the result, not the gradual effect, of the pressure. More recently, an 

 instrument invented by Purkinje, and which bears the discoverer's name, has 

 been made use of for this purpose, and also an improved form by Shiek. The 

 gradual effect of the pressure can very readily be observed under the microscope 

 with this auxiliary. The value of this instrument has been overrated by Purkinje, 

 but has been very unjustly rejected altogether by Meyen. It is, perhaps, the only 

 means of distinguishing a small globule from a vesicle, which latter, without 

 having any real existence, occupied so prominent a position for a long time in 

 botanical works. 



c. Chemical. 



The different phenomena which a substance presents upon the application of 

 chemical reagents are of the highest importance for the formation of our opinion. 

 Indeed, it very frequently occurs that we are obliged to determine substances 

 according to their chemical nature, which, enclosed in organised bodies in a 

 small quantity, cannot mechanically be separated from them, at least not in such 

 a manner as to enable us to institute a chemical analysis of them. There is no 

 other means left to us, therefore, but to use such agents under the microscope 

 itself.* The principal of such reagents are : 



1. Tincture of Iodine. Particularly useful for rendering visible very trans- 

 parent objects, and for the determination of various vegetable substances. 



2. Sulphuric Acid. For the destruction of certain parts. 



3. Fixed Oils. The best of all is the oil of almonds. Volatile oils, oil of 

 lavender, alcohol and ether, and Canadian balsam, in order to render objects 

 transparent, and to dissolve species of fat and resin, and bring substances into a 

 state of coagulation ; as, for instance, albumen. 



4. Solutions of Sugar, Gum, and Albumen. In order to prevent endosmosis, 

 and the changes of form consequent thereon. 



5. Solution of Caustic Potash. For the destruction of certain parts. 



6. Acetic Acid, Nitric Acid, Muriatic Acid. In order to dissolve many sub- 

 stances. 



The reagents enumerated last (under No. 6.) ought to be avoided as much as 

 possible, when the achromatic microscope is made use of; and at all events the 

 object ought to be covered by a glass plate, since the evap6rating acids very 

 readily produce an effect upon the very susceptible flint-glass. 



d. Physical. 



It may occasionally happen to be of interest to observe the effect of heat and 

 electricity upon certain objects under the microscope. Peculiar contrivances are 

 necessary for this. The application of heat requires glass plates that have pre- 

 viously been thoroughly annealed, which may be heated by means of a small 

 spirit-lamp at one end, or by means of very thin glass plates loosely put into a 

 brass frame, and heating the latter. For the observation of electrical effects we 

 have a small peculiar stage, on both sides of which two small forceps hold move- 

 able little pieces of a glass tube, through which wires are drawn. These wires 

 reach the stage at one end, and have a small hook at the other, in order to attach 

 the conducting wires. 



Many errors, which but too frequently occur in botanical works, will be avoided 

 if the auxiliary means enumerated above are applied, and if attention is paid to 

 the cautions and hints communicated. Once for all, however, I must repeat the 

 fundamental rule, that he who wishes to observe with success, must observe 

 frequently and with the most profound attention : by observing this rule, he may 

 gradually learn to see, for seeing is a very difficult art. 



Woodward, on polarised light ; and also the Transactions of the Microscopical Society. 

 THA NSI.ATOR. ] 



* J. Vogel, Anleitung zum Gcbrauch des Mikroskops, &c. 



