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 eye-pieces ivhich are intended 

 for use with stands not made by us, it is advisable to send ^ls 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 "Ueber Verbesse- 

 rungen des Mikroskops mit Hilfe neuer Arten optischen Glases" 

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

 pp. 107 128) and to Dr. CZAPSKI'S work on the "Theorie der optischen 

 Instrumente", Breslau, 1893. 



The peculiar characteristic which, from the optical point of view, distingui- 

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

 microscopes lies in the simultaneous realisation of two conditions of concentration 

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

 combination, 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 so-called secondary 

 spectrum left unconnected 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 

 is 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 equally 

 sharp. The quality of the image will therefore be always uniformly perfect whether white 

 or monochromatic light be employed for illumination. 



Again, in the older series complete colour correction is obtained for one zone 

 of the objective only, a marked deterioration being observable towards the margin and the 

 centre of the aperture, whilst in the apochromatic lenses uniformly perfect colour 



