586 APPENDIX. 



of such colours by placing the object exhibiting them, even when it does so, in the 

 centre of the field, where naturally perfect achromatism takes place in ail posi- 

 tions, and should attribute such colours to the object itself, after it has been 

 found impossible to get rid of them by every precaution. The assertion of some 

 observers, that the inner circle of the pores in the cells of the Coniferce (the real 

 pores) occasionally appears to be of a green colour, furnishes an illustration of 

 this kind of error. 



Under this head, also, belong certain aberrations of form, which are caused by a 

 defective method of placing the object within the correct focal distance : thus, lines 

 will appear double, or of a certain breadth, which, on being accurately placed, 

 present simple lines, or as sharp lines without any apparent breadth. This is 

 probably a phenomenon of diffraction. In this instance, neither the apparent 

 breadth nor the reduplication of the lines belong to the object itself, as these phe- 

 nomena disappear when perfect distinctness of the image is obtained by means of 

 a correct way of placing the object. An instance of optical illusion may be found 

 in Mirbel's treatise, " Nouvelles Notes surle Cambium" (Archives du Museum 

 d'Hist. Nat., 1839. p. 303. etseq.). He there makes mention of cells (pp. 306, 

 238, Table xxi. fig. 3. and fig. 6.), the walls of which appeared to be marked on 

 a transverse section by transverse striae, which, however, disappeared on the 

 examination of a longitudinal section, then presenting longitudinal striae. I have 

 frequently observed this phenomenon, and have no doubt of its being an optical 

 illusion. Mirbel has been rather too free with his stria? in the plates mentioned, 

 as we never see more (less) than four, viz. the upper and lower cut surface of the 

 cell and two lines. The proof of its being an optical illusion is made evident by 

 the fact, that we can never obtain a sight of two of these lines alone by any change 

 of the focus. They either all four appear, or only the upper, or only the lower 

 surface. I do not find that any one has as yet directed attention to this pheno- 

 menon, and much less has any explanation been given of it. It is unquestionably 

 certain that the object only lies in the correct focal distance when its image 

 appears to be most distinctly and accurately represented. But the differences in 

 distinctness and accuracy are so fine, that they frequently hardly become per- 

 ceptible to the most practised eye. The rule may, therefore, be better given by 

 saying that the correct focal distance has been found when the image appears 

 smallest, and when the dimensions of all the parts, and of all the lines and points 

 of which it is composed, exhibit the smallest size. It will always be found that 

 the greatest accuracy and distinctness exist in that case, where each line and each 

 point appear the darker, the smaller and the narrower they are. There are 

 probably many other conditions which embarrass our judgment with regard to 

 microscopical objects, but at present none else have come within my own cognisance. 

 We find, alas ! no information at all respecting these things in the writings of 

 natural philosophers, because no one has as yet occupied himself with the theory 

 of microscopical observation. 



Another preparation, besides the knowledge of the optical facts, is necessary for 

 the task of enabling us to distinguish the phenomena that do not in reality belong to 

 the object under observation. These optical facts belong to the image which the 

 object-glass produces, and only occur in the compound microscope. But there are 

 a great number of phenomena which are connected with the real object on the 

 stage, but yet do not belong to the real object of our observation. These likewise 

 interfere during the use of the simple microscope. We must be thoroughly ac- 

 quainted with these phenomena before we can proceed with any microscopical 

 examination with any hope of success. The requirement, indeed, in this case would 

 be to make ourselves masters from personal observations of all objects already 

 examined, before we proceed to the examination of a new object. But a cursory 

 glance at the results already attained by the microscope shows the impossibility 

 of satisfying such a demand. We must therefore limit ourselves, and instead of 

 so comprehensive a demand, we will state two more practicable, but quite in- 

 dispensable requirements. The first consists in the necessity of making ourselves 

 acquainted with the general phenomena that may possibly occur on every examina- 

 tion, before we avail ourselves at all of the microscope for our researches ; and 

 the second consists in the necessity of studying accurately, previously, every- 

 thing that is already known respecting the special object of our examination. We 



