1884.] 



MICROSCOPICAL JOURNAL. 



195 



scarcely exceeds 112,000 lines to the 

 inch,* which should theoretically be 

 attained with an aperture of 1.16 

 but the optical construction does not 

 permit of the attainment of theoretical 

 perfection. The highest numerical 

 aperture to be obtained from the op- 

 ticians is about 1.43. 



Before entering upon another and 

 perhaps more generally interesting 

 part of the subject before us, I ^vould 

 wish to draw your attention to a fact 

 that cannot be too strongly brought 

 forward. From vv^hat has been said 

 concerning the formation of images of 

 minute markings b}' the decomposi- 

 tion of the light and the production 

 of spectra, it will be readily inferred 

 that whatever objects will form iden- 

 tical spectral images, no matter how 

 different the structui^e of the objects 

 themselves may be, their appearance 

 in the microscope will be the same. 

 As a single example to illustrate the 

 practical bearing of this fact, we may 

 consider the resolution of any finely- 

 marked diatom. By changing the 

 illumin ition the dots may become 

 lines, or the lines in some cases may 

 be made to appear twice as close or 

 half as close as they really are. Time 

 does not permit me to enlarge upon 

 this subject as I would wish, but it 

 has an important bearing upon our 

 interpretation of the revelations of 

 the microscope. 



We have already seen that there is 

 a theoretical as well as practical limit 

 to the power of a microscope to sepa- 

 rate fine lines. This limit is deter- 

 mined first by the numerical aperture 

 of the objective, and, secondly, by the 

 magnifying power of objective and 

 ocular combined, which in every case 

 must be sufficient to separate the lines 

 in the image until they subtend an 

 angle large enough to enable the eye 

 to distinguish them. This amplifica- 

 tion attained, supposing the resolving 

 pov\^er of the objective to be tested to 

 its highest limit, no further amplifica- 



*i2o,ono hns been claimed, but we are not sa isfied 

 with the evidence in support ol this, rather improbable, 

 accomplishment. 



tion can possibly reveal any finer de- 

 tails in the image. Nevertheless, a 

 slight increase is desirable to render 

 the details more distinct, as all who 

 work with delicate test-objects well 

 know. There is, therefore, a well- 

 defined relation between numerical 

 aperture and amplification which ren- 

 ders it possible to define the limits of 

 adequate and desirable amplification 

 for every objective. A recognition 

 of this fact will show that the use of 

 oculars of excessively short focal 

 length, or high power, must be ex- 

 tremely limited in scientific investi- 

 gation. 



Passing now^ from the microscope 

 itself, w^hat has been said will enable 

 us to appreciate the value of certain 

 accessories for illuminating objects 

 under investigation. It was not many 

 years ago that the most valued, and 

 likewise the most costly, apparatus for 

 illuminating objects for high-pov\^ers 

 was the achromatic condenser, made 

 very much like an objective, with a 

 small front lens, and a complicated 

 system of diaphragms beneath. For- 

 tunately such costly appliances are 

 no longer necessary, and although 

 achromatic condensers are still made 

 their form is quite difterentfrom those 

 in general use ten years ago. 



For the scientific student there is 

 no device more generally useful than 

 the Abbe illuminator, with its full 

 complement of diaphi'agms. It is not 

 strictly achromatic, but that is of no 

 consequence. It is not costly com- 

 pared with other forms, and is well 

 adapted for universal use. It gives 

 the observer complete control over 

 the light, and is a great aid in all 

 delicate observation. I feel like dwell- 

 ing longer upon this subject to urge 

 the use of this apparatus upon our 

 scientific students. I am aware how 

 many of them affect to despise all 

 accessories. It is because they do 

 not know their value. It will not do 

 to say such things are of no use merely 

 because they have not felt the need of 

 them, for such persons do not know 

 of what they speak. The true stu- 



