578 APPENDIX. 



We have still to answer the question, Whether the simple or the compound 

 microscope is preferable for scientific investigations ? I must decidedly declare 

 myself in favour of the latter, and that from the following reasons. Ceteris 

 paribus, the simple microscope injures the eye much more than the compound, 

 because the strength of the light (which is quite independent of the strength and 

 clearness of the image, and ought to be clearly distinguished) is more intense, and 

 strikes a smaller portion of the retina, and therefore causes a greater inequality 

 in the excitement of the optic nerve ; secondly, on account of the great incon- 

 venience of the very short focal distance in higher powers ; thirdly, because we 

 can obtain higher powers, with the same mathematical accuracy, through the 

 compound ; and, lastly, because all the objections which were formerly urged 

 against the compound microscope have been removed. Habit will also do much ; 

 but on comparing the observations of the last twenty years, it must undoubtedly 

 be admitted that the discoveries and observations which have advanced science 

 have been made by the compound microscope, with the exception of those of 

 Robert Brown of a man who, because he is perfectly sui generis, and has not 

 his equal, should not be compared with ordinary observers. Thus much with 

 regard to the value of the instrument. Previously, however, to proceeding to the 

 method of observation, I must touch upon two points demanding careful con- 

 sideration, because they frequently exercise great influence on scientific results, 

 namely, the measurement and the illumination of objects. 



a. The determination of the magnifying power of a microscope was of much 

 greater importance in former times, before we possessed a suitable apparatus for 

 determining directly the size of microscopical objects, than now. Formerly they 

 divided the apparent diameter of the object by the number of the magnifying 

 power, and thus discovered the size of the object itself. This mode of pro- 

 ceeding is, of course, too primitive to have any scientific value, and has conse- 

 quently been abolished long ago. Nevertheless, it is of great interest to know, in 

 many cases, of what degree the magnifying power is of which we avail ourselves. 

 Good opticians generally attach an index to their instruments* for the magnifying 

 power of the different combinations. But as considerable errors will occasion- 

 ally be made by them, it is necessary that the observer himself should be able to 

 ascertain the magnifying power of his instrument. This is attended with no great 

 difficulty with regard to the simple microscope ; it is also easy, after some 

 practice, with the compound microscope. All that is required for it is a measure 

 inscribed in black on ivory, or on very white paper, which gives lines, and a glass 

 micrometer, which contains the same lines divided into optional parts (if intended 

 for a very strong magnifying power, into at least sixty). The glass micrometer is 

 then laid under the microscope, and on arranging it so that the divided lines may 

 be seen distinctly, the measure is laid on the stage of the microscope. On 

 looking now with one eye through the microscope, with the other on the measure 

 beside it, which in most of the newer instruments will be within the distinct 

 range of sight, owing to the length of the tube, both measures may bs compared 

 the one with the other, which, after some practice, is very easy : thus, if we have 



author wrote (1845), instruments had been made by the great English makers, Ross, 

 Powell, and Smith, which have certainly never been surpassed, if they have been 

 equalled, by continental makers. Dr. Schleiden's observation on English observers is, 

 I fear, the reverse of the truth : we have plenty of microscopes, and those the best in 

 the world ; but we have had but few observers. Our microscopes have been used rather 

 as playthings than as the instruments of profound philosophical research. Let us hope, 

 however, that this reproach will soon be wiped away. Already, through the efforts of 

 the Microscopical Society of London, which was founded in 1839 to cultivate a branch 

 of scientific inquiry which the older societies neglected to encourage, improvements in 

 the microscope have been made, and a knowledge of its powers and mechanical arrange- 

 ment diffused, which are bearing fruits not only in its own transactions, but in the 

 transactions of some of our other scientific societies. Those who would wish to study 

 the history of the microscope, and all that relates to it, I must refer to the admirable 

 treatise on the microscope, by Mr. John Quekett, secretary to the Microscopical 

 Society. TRANSLATOR. 



* Schiek's statements are generally very accurate; Plossl's, however, are almost all 

 erroneous, and, it may be said to his honour, almost all too low. 



