On Improvements of the Microscope. By Prof. E. Ahhe. 33 



tance between its lower focus and the upper focus of the objective, i. e. 

 the " optical " tube-length. One and the same eye-piece will therefore 

 give different eye-piece magnifying powers— just as it gives different 

 values of the total magnifying power — according as it is used with a 

 short or long tube, and these vary in exactly the same proportion as the 

 distance A. Such alterations, which are produced in using the Microscope 

 by pushing in or drawing out the tube, can be determined as regards 

 their influence upon the eye-piece magnifying power just as readily as 

 their influence on the total magnifying power. If it is known that 

 the magnifying power ascribed to an eye-piece refers to an optical tube- 

 length of say 180 mm., it is also known that an elongation of the tube by 

 20 mm. will raise the magnifying power of the eye-piece in the ratio of 

 180 : 200. In order to take account of all such alterations, it is only 

 necessary that there should be given by the optician, or determined by 

 the observer himself, the distance between the upper focus of the objec- 

 tive and the under focus of the eye-piece for that length of tube which is 

 adopted as the normal length. If this is unknown it will be also 

 impossible to determine the change produced in the total magnifying 

 power of the Microscope by a change in the length of tube. 



Contrivances fok Projection. 



To gain a complete idea of the advantages secured for photomicro- 

 graphy by the apochromatic objectives, special attention is finally directed 

 to the means of projecting the image. 



The methods hitherto employed for this purpose are all beset by 

 considerable drawbacks. The simplest and apparently the surest plan, 

 the direct projection of the image upon the photographic plate, always 

 leads in the case of objectives of considerable aperture, to a deterioration 

 of the image by spherical aberration as soon as the distance of the plate 

 is much greater than the normal length of tube for which the objective 

 is corrected. If the distance of the plate is great, these aberrations 

 which are produced by the altered path of the rays in the objective 

 cannot be entirely removed even by the correction-adjustment. It is 

 true that these sources of error are eliminated by the projection of 

 images with an eye-piece; but the ordinary eye-pieces, especially the 

 unachromatic, lead for their part to other considerable errors, since 

 they largely increase the difference of focus of the chemical rays. The 

 use of an achromatic dispersive lens (amplifier) in place of an eye- 

 piece, which has hitherto led to the relatively best results, introduces, 

 apart from other objections, minute and troublesome manipulation in 

 the adjustment, if a good correction of the objective is to be secured. 



The method employed by the author, which disposes of these defects, 

 seeks to produce the objective image under the same conditions and at 

 the same point of the tube as with eye-piece observation, and then 

 to project this image upon the plate (or a screen) by means of a system 

 of lenses accurately corrected for spherical and chromatic aberration, 

 which can be focused to the objective image in the tube. 



This method makes it absolutely certain that the objective as 

 corrected (by means of the correction-adjustment) for the eye, remains 

 m exactly the same condition when the image is projected, and that 



1887. D 



