298 SUMMARY OF CURRENT RESEARCHES RELATING TO 



tially enhance the performance of the Microscope in the whole ; for, 

 with the present constitution of the objective, magnifying powers are 

 attainable with which the smallest detail that can be represented is 

 distinctly visible ; the progress will merely consist in obtaining in equal 

 perfection the same amplifications that we now have at command hy 

 means of relatively lower power objectives. But even this would be a 

 matter of great practical importance if we should succeed in materially 

 surpassing the present performance, so that, for instance, the strongest 

 magnifying power, for which we now use lenses of 1 mm. and less focal 

 length, would be at least as perfectly obtained with objectives of from 

 3 to 4 mm. Not only would the great difficulties be removed, which are 

 now attendant upon work with high magnifying powers, in conse- 

 quence of their too short working distance, but it would be no less an 

 advantage that every objective would offer a greater latitude for useful 

 magnifying power. Even with regard to purely optical perfection, the 

 production of higher magnifying powers by stronger eye-pieces, instead 

 of stronger objectives, would be a decided gain, by lessening certain 

 faults in the image which impair its clearness outside the axis. The 

 aberrations outside the axis (erroneously attributed to the convexity 

 of the field) which in objectives of large aperture are always but 

 very imperfectly corrected, vary for the most part with the square of 

 the distance from the axis, for which reason their obnoxious influence 

 will, with the employment of stronger eye-pieces, diminish more 

 quickly than the magnifying power increases. Possible apprehensions 

 of other drawbacks which might attend the use of stronger eye-pieces 

 are groundless, for, if it should become necessary, combinations of 

 lenses could be constructed, by which any high magnifying power 

 that may be desired could be as conveniently obtained as with the 

 present eye-pieces. 



In judging of the ways and means which are open, according to this 

 view, for the perfecting of the Microscope, we must consider the various 

 sources of error which spring from its deficiencies. — At the present 

 day an extraordinary perfection is attainable in the technical accom- 

 plishment of objectives. With technical aptitude and a rational method 

 of work we can correctly produce given curves, even in very small 

 lenses, up to a few thousandths of the radius. The irregular deviations 

 of the surface from a strictly spherical form, however, can be restricted, 

 when necessary, in their absolute quantity to small fractions of the 

 wave-lengths and the centering of the separate surfaces can be exactly 

 executed with exceedingly small deviations. With the exception, 

 perhaps, of the strongest objectives, which, in consequence of their 

 very small dimensions, allow of only uncertain means of measuring 

 and proving, we can see the unavoidable errors diminishing almost to 

 the vanishing point. The actual imperfections which are seen in the 

 dioptrical working of the Microscope of the present time must chiefly 

 be referred, therefore, to the imperfect correction of the aberrations. 



The study of the conditions which must be fulfilled for the perfect 

 correction of the chromatic and spherical aberrations in an objective 

 shows two drawbacks not to be OYercome by the practical optics of the 

 day. 



