Si/siem of Homogeneous Immersion. By Prof. E. Abbe. 261 



essentially necessary if the capacity of the new objectives is to be 

 fully utilized, it is important to have a simple means of regulating 

 the refractive and disjDersive powers of the fluids in their relation to 

 the corresponding factors in crown glass without having to employ 

 special measuring apparatus. For this purpose Mr. Zeiss furnishes 

 with each objective a small glass bottle with parallel sides, to the 

 glass stopper of which is cemented a crown glass equilateral prism. 

 This test bottle may be used in preparing the combined fluids, 

 and by viewing the vertical bar of a window frame, &c., through 

 both fluid and prism the difierence between the fluid and crown 

 glass, both with respect to refraction and dispersion, may be at 

 once seen. The deflection of the image of the vertical bar in 

 passing through the prism, and the width of the coloured border, 

 gives both these elements at a glance and with an exactness which 

 is quite sufficient. 



In the practical use of the new objective there are two further 

 points to be specially noticed. The first is its dependence upon the 

 length of the tube. The abolition of the cover-glass correction in 

 these objectives, which is acknowledged by all observers to be an 

 extraordinary advantage in manipulating the lenses with ease and 

 certainty, nevertheless deprives the observer of a convenient means 

 of compensating within certain limits the influence of diflerent tube- 

 lengths upon the aberrations.* 



The objectives can therefore only be used with the length of 

 tube for which they are originally adjusted, and they are so sensi- 

 tive on this point (especially the lowest power) in consequence of 

 the large angular aperture, that a deviation of a very few centi- 

 metres in the length of the tube produces visible changes in the 

 condition of the correction. A draw tube to the Microscope affords 

 therefore a very simple means of regulating according to the 

 observer's own judgment, the ultimate more delicate adjustment 

 of the correction, and also enables him — until some better immer- 

 sion fluid is found — to compensate any small defect in the refraction 

 of cedar- wood oil, which may be noticeable when very thick or very 

 thin cover-glasses are used. (As lengthening the tube produces 

 spherical over-correction, and shortening under-correction, it follows 

 that the former corrects a very thin covering glass, and. the latter 

 one of more than ordinary thickness.) 



* Dispensing with the correction-adjustment in the manufacture of sucli objec- 

 tives is a matter of small moment in itself when compared with the other technical 

 requirements which are met by it. An essential benefit arises, however, from 

 the simplification of the mechanical construction, in so far as it would scarcely 

 be possible in a combination of lenses with movable parts to get the lenses centered 

 as perfectly and durably as is possible in the case of a fixed combination : and in 

 the present instance this appears an iadispensable condition on account of the 

 sensitiveness of the large aperture to the slightest defect in centering. Looking 

 at tiiis circumstance, it would be most unadvisablc to provide such objeotives with 

 correction collars. 



