30 Transactions of the Society. 



of rays of not less than the eleventh order; three different kinds of 

 rays are brought under strict conditions by the removal of colour- 

 deviation, including secondary dispersion ; six by the elimination of 

 spherical aberration and its chromatic difference with such large aperture ; 

 and, finally, tivo more by the production of equal magnifying power for 

 the different zones of the free opening. To this corresponds the 

 enormous size of the free opening which is employed with Microscope 

 objectives as compared with telescope objectives ; whereas in the latter 

 an opening which is only the tenth j;ar^ of the focal length is quite 

 exceptional, at least in the larger instruments, the Microscope objective 

 of the present time requires openings the diameter of which are 2 ■ 8 

 times the focal length. 



Eye-pieces. 



The following out of the rules above indicated for the construction 

 of objectives, has naturally given rise to increased requirements in the 

 case of the eye-piece, which have not been hitherto taken into account, 

 and the satisfaction of which has led to several new contrivances. With 

 respect to these, the following considerations have been determinative. 



In the first place, if the advantage of the new objectives which was 

 pointed out above is to be realized, the eye-pieces to be combined 

 with them must conform to the condition that they should be suflS.- 

 ciently achromatic in regard to the distance at which the differently 

 coloured rays are united, whilst with respect to magnifying power they 

 should behave as strongly over-corrected lenses, and the degree of over- 

 correction should be quite definite, being determined by the correspond- 

 ing deviation of the objectives of large aperture. To satisfy this quite 

 unusual requirement, that the focal length of such an eye-piece must 

 differ for different coloui's in exactly the of)posite way to an ordinary un- 

 achromatic lens, no special difficulties are experienced in practical optics, 

 and the object may be attained in the different types of construction by 

 known means. Eye-pieces of this kind compensate for the chromatic 

 difference in magnifying power of the objective, and they do this at the 

 same time for different objectives if, as was said above, the latter are 

 equahzed so that this difference is the same for all. Having regard to 

 this property, the eye-pieces in question are called compensation eye- 

 pieces. 



In the second place the demand, that with the new objectives a high 

 magnifying power in the eye-piece should be provided for systematic 

 work, requires a considerable alteration in the construction of the 

 stronger eye-pieces so as to free them from the disadvantages by which 

 they are at present beset. The types of eye-piece which have been 

 hitherto used, both the ordinary Huyghenian and the different achromatic 

 eye-pieces, if their focal length falls much below 25 mm., require eye- 

 lenses of very small diameter, and moreover, with them the eye-point 

 lies very close to the lens. On this account the observer must bring his 

 eye inconveniently near, and in particular the use of the camera lucida 

 is prevented. By a type of construction which is essentially different 

 from the forms in use, the compensation eye-pieces may be rendered quite 

 as convenient for the higher powers as is the case with the weak and 



