178 M. Steinheil's New Micrometer. 



ing to two or three minutes are unavoidable, even with the 

 greatest attention. 



Addition. — It may be desirable to mention, that the micro- 

 meter-apparatus here described may be made use of also as 

 an achromatic microscope, if the object to be viewed is brought 

 into the focus of the small object-glass instead of the micro- 

 meter-plane. But it will be necessary, for the purpose of illu- 

 minating it and of rendering it more commodious, to give to 

 the whole a more convenient construction. The small object- 

 glass might (as shown in fig. 2.) be screwed into a tube which 

 could be clamped to the telescope, and should in such case 

 have two incisions in the focus of the small object-glass, in 

 order to receive the frame holding the object which is to be 

 viewed. Although this object may not be perfectly in the 

 focus, its image may be rendered distinct by changing the po- 

 sition of the eye-glass in the telescope. The ring, represented in 

 the figure, will serve to support the telescope and microscope. 



The power of this microscope is, independently of its abso- 

 lute size, the more considerable, the greater the ratio of the 

 aperture of the second object-glass to its focal length. It 

 would, therefore, be more advantageous to use object-glasses 

 composed of more than two lenses. Perfect object-glasses of 

 four lenses would, as far as spherical aberration is concerned, 

 admit of almost a double aperture, and, as the loss of light by 

 refraction and reflection is very small, their effect would be 

 nearly double. The image would likewise gain in distinctness, 

 as it would be possible to destroy the chromatic aberration for 

 rays incident at a distance from the axis. 



I am now engaged in calculating such object-glasses, and 

 M. Merz (the director of the establishment of Utzschneider 

 and Fraunhofer) having promised the execution of them, I hope 

 to be able at some future period to communicate the result of 

 my labours. 



If we are satisfied with a double lens, the effect of this 

 microscope is somewhat less than that of Fraunhofer's ; be- 

 cause, in the latter the object may be brought nearer to the 

 object-glass than its focal distance. The advantage which it 

 may, however, claim over it, is the absolutely greater focal di- 

 stance of the object-glass of the microscope, in consequence 

 of which the whole of an object of some thickness may be 

 viewed with distinctness at the same time. It seems to deserve 

 the attention of travellers, on account of the ease with which 

 every one can thus convert his telescope into a microscope. 

 Different eye-glasses would, of course, change the field of 

 view and the magnifying power of such a microscope. 



I must 



