OBJECT-GLASSES. xv 



APERTURE," in which also is contained a list of the various apertures of the best glasses, so 

 that the approximation in the cases of any glass to these magnitudes, will afford an indi- 

 cation of its quality. 



The following remarks may perhaps assist in guiding the judgment in regard to the 

 selection of an object-glass : 



1. Large angular aperture is of less importance in the case of a low than of a high power. 



2. Large angular aperture is neither requisite nor advantageous in physiological and 

 medical investigations in general. 



3. Whether a glass of larger aperture will exhibit any further structure than one of less 

 aperture has already done, can nearly always be predicted from other means. 



4. Object-glasses of high power and large angular aperture require to be brought very 

 close to the objects viewed, which is a great disadvantage, rendering them useless for gene- 

 ral investigations. 



5. In regard to objects requiring large angular aperture for exhibiting their structure, 

 much depends upon the management of the light -, so that a glass may fail in exhibiting 

 certain parts of structure in the hands of one of but little experience, whilst in the hands of 

 another it may show them distinctly. Hence the direct measurement of the angle is best 

 to determine what a glass is capable of exhibiting when properly used. 



6. The markings on the Diatomaceae were discovered by the aid of foreign glasses of 

 small angular aperture. 



7. Almost all the investigations which rendered the microscope an instrument of science, 

 have been made with foreign object-glasses of small aperture ; and where these have been 

 found faulty, the fault has arisen mainly either from judging of structure by simple inspection, 

 or substituting analogical reasoning for observation. 



8. The English object-glasses are very expensive; but they are incomparably superior to 

 the foreign in every respect in denning power, in penetrating power, in the centring of 

 the lenses, in the existence of an adjustment for varying thickness of glass, and in general 

 perfection of workmanship. These advantages tell principally in the higher powers. An 

 English glass when used with the highest eyepiece will still define better than nine-tenths 

 of the foreign glasses with the lowest eyepiece. 



As a complete set of English object-glasses is very costly, many persons will perhaps 

 prefer having some English and others foreign. Under these circumstances, the higher 

 powers should be of English and the lower of foreign manufacture. 



It might be objected, that the structure of many of the very minute and delicate 

 objects examined by our continental neighbours have been erroneously described ; and this 

 would be a fact. But this arises from unacquaintance with certain precautions essential to 

 the proper use of high powers ; and the same errors have been committed by our own 

 countrymen, from the same cause, even with the finest object-glasses which have been made. 



The student may perhaps find himself perplexed by the conflicting statements made by 

 different renowned observers in respect to object-glasses. The well-known Schleiden says 

 that only a magnifying power of about 500 diameters is useful for scientific purposes ; that 

 with our present microscopes, we may see whatever we like with a power of 3000 j and that 

 only the amplification of an object to the extent of 280 or 300 diameters is produced by the 

 object-glass, all beyond this being effected by the eyepieces with an almost total loss of light. 

 Now these statements may be very true in regard to German object-glasses as used by the 

 Germans ; but they do not apply to the English object-glasses as used here. The highest 

 English object-glasses (the one-twelfth of Ross and the one-sixteenth of Powell) will show 

 minute objects with a power of 600 diameters with the lowest eyepiece, as clearly and well 

 defined as the German glasses of 1-inch focus will show larger objects ; and we feel convinced 



