580 APPENDIX. 



by the naked eye, is readily and distinctly recognised as soon as it has been made out 

 by the telescope. On the same principle, it only requires a very low magnifying 

 power (perhaps 100 times) in order to be perfectly convinced of most facts in 

 the anatomy of plants. The very high magnifying powers are, to a great extent, 

 useless for morphological researches, and, with respect to these, there is still so 

 productive and so little cultivated a field of investigation before us, that we may 

 safely promise scientific immortality to any one who undertakes such researches 

 with sincere industry and honest zeal, even with the most simple instruments. 

 There is so much to be done in this department, that it would be difficult to 

 avoid discovering something new. Skill in the preparation of objects, practice, 

 and natural talent, are here of much greater importance than expensive instru- 

 ments. I would here particularly draw attention to the pocket microscopes, 

 which are now manufactured by Dr. Korner, in Jena. They are packed in a 

 little case, the cover of which serves as a stand. The moveable stage is fur- 

 nished with a screw for the purpose of affixing it during the preparation of an 

 object, and with a round plate of glass, in order to moderate the light from below, 

 or to arrest it altogether. In a moveable arm, four double lenses may be placed, 

 which afford a clear and beautiful magnifying power of 15 120 diameters. 

 This instrument is quite sufficient for all entomological, pharmacognostical, and 

 botanical researches, and even for perfecting satisfactory anatomical observations. 

 This instrument, with case and apparatus, only costs three Friedricsd'or, or se- 

 venteen dollars Prussian currency [about 21. IQs. sterling], which would unques- 

 tionably be better laid out than if we were to purchase hay with it, or, in other 

 words, 300 or 400 specimens of dry plants.* 



The determination of the absolute size of very small objects is far more im- 

 portant than the determination of the magnifying power of the microscope. Ac- 

 curate observers, long ago, sought for means to accomplish this ; Leeuwenhoek, 

 for instance, used clean grains of sand : having first ascertained how many of 

 them went to a line, he strewed them among the microscopic objects, and thus 

 determined the size of the latter by comparison. Other small substances, for 

 instance, pollen, were subsequently used for this purpose. After the discovery of 

 the transverse striae upon the muscles, they were recommended for the same pur- 

 pose, likewise the blood-globules of different animals. But all these experiments 

 are of little value in a scientific point of view. The production of real microsco- 

 pical measuring instruments was, therefore, early thought of. The oldest of them 

 was the so called glass micrometer, namely a smooth little plate of glass, in which 

 very fine divisions were cut by a diamond. Dollond especially distinguished 

 himself by the manufacture of excellent and accurately finished micrometers. 

 Within a more recent period they have been made by all good opticians. Che- 

 valier manufactures micrometers in which the millimeter is divided into 500 parts, 

 or the line into about 1000 parts. But these micrometers have nevertheless, 

 some important drawbacks. Even with the best diamond, the splintering of the 

 edges of the drawn lines can scarcely be avoided. In many cases, also, a glass- 

 micrometer is not available at all. It is impossible, with very small objects, or 

 with very high magnifying power, to keep the object and the divisional lines of 

 the micrometer simultaneously within the same focus, which renders an accurate 

 measurement almost impracticable. Objects also, which must necessarily lie in 

 the water, in order that they may be brought under the microscope, cannot well 

 be measured by the glass micrometer, as the small divided lines are filled up with 

 water, and are thus rendered almost invisible. 



The screw micrometer, which was first applied by Frauenhofer, is now made 

 use of for all real scientific researches, and is generally provided with all the larger 



* I am not aware that a good working microscope could be obtained in England at 

 the above price ; but a very serviceable instrument may be obtained of any of our great 

 makers at a moderate sum compared with the price given for the highest powers, ac- 

 companied by the complicated accessories of a complete apparatus. It should be 

 recollected that Ehrenberg, with a thirty-shilling microscope, produced his great work 

 on the Infusoria, a work with which British microscopy has nothing to compare, 

 although it has spent thousands of pounds annually on its instruments. TRANSLATOR. 



