= 12 = 



Abhandlungen I, Jena 1904, pp. 267, 354, 365, 360, and CZAPSKI, Theorie der 

 optischen Instrumente, 2 nd Edition, Leipzig 1904, pp. 245, 254.) 



The mounts of all objectives for monocular observation are provided with 

 the English standard screw of about 20 mm ( 3 /4 in.) external diameter. 



The name of our firm is engraved on the mounts of all our objectives; on the 

 apochromatic objectives are further engraved the aperture, focal length and tube- 

 length for which they are adjusted, and on the achromatic lenses their distinguishing 

 letters. 



When ordering objectives and oculars which are intended for 

 use with stands not made by us, it is advisable to send us the 

 tube in question for adaptation. 



*Apoehromatie Objectives. 



We cannot here do more than briefly describe the essential features of the 

 apochromatic objectives and must refer those who wish to make a more com- 

 plete study of the scientific aims and principles which govern the construction 

 of these lenses, to the paper of Professor ABBE, entitled "Uber Verbesse- 

 rungen des Mikroskops mit Hilfe neuer Arten optischen Glases" 

 (Sitzungsberichte der Med.-naturwissensch. Gesellschaft zu Jena, 1886, Vol. XX, 

 pp. 107 125, ABBE, "Gesammelte Abhandlungen" I, pp. 450 472), and to Dr. 

 CZAPSKI'S work on the "Theorie der optischen Instrumente", Breslau 

 1893, 2 nd Edition, Leipzig 1904. 



The peculiar characteristic which, from the optical point of view, distinguishes 

 the objectives of this series from all lens systems previously applied to micro- 

 scopes lies in the simultaneous realisation of two conditions of concentration of 

 rays, which conditions had hitherto not been fulfilled by any other optical system, 

 viz: 1) the union of three different colours of the spectrum in one point of the 

 axis, that is to say, the elimination of the secondary spectrum left uncorrected 

 in the older achromatic lenses, and 2) the correction of the spherical aberration 

 for two different colours, in contradistinction to the usual correction for one colour, 

 that in the brightest region of the spectrum, only. 



With all optical systems constructed up to 1886 the greatest sharpness of the image 

 was limited to one particular colour of the light transmitted, while the other rays give more 

 or less confused images, appearing partly as colour fringes surrounding the sharpest image 

 and partly as a general haze spread over the whole field. But with the apochro- 

 matic lenses the images are for all colours of the spectrum nearly 



