850 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



carried out exactly on the data furnishod by those calculations, with 

 the strictest check on all the elements in tho various stages of manu- 

 facture, and without any subsequent empirical touching up. 



In the list given below the objectives are arranged according to 

 their aporture. In the second column are the different focal lengths, 

 while the third column gives the corresponding amplification obtained 

 with tho objectivo (the quotient of the conventional distance of dis- 

 tinct vision, 250 mm., divided by the focal length of the objective). 



The objectives are constructed according to order, either for tho 

 Continental tube-length of 160 mm. or for the English of 250 mm. 

 (or 10 in.). Tho three dry objectives of 6, 12, and 24 mm. focal 

 length are, however, made exclusively for the English tube-length, 

 as these objectives are not adapted for the Continental form. The 

 tube-length is measured from the upper surface of the setting of the 

 objective to the upper margin of the body-tube on which the eye- 

 piece rests. 



Great care must bo taken to preserve the correct tube-length, as 

 any deviation materially injures the performance of the objectives, 

 particularly those for homogeneous immersion. 



The settings of all the objectives are engraved with the name of 

 the firm and also with the aperture, focal length, and length of the 

 body-tube, for which they are adjusted. In ordering it is desirable 

 that these three points should be specified so as to avoid any mistake as 

 to the particular objectivo required (thus: Apochrom. 1-30, 2-0 mm., 

 short tube). 



The apertures given are the guaranteed minimum values; the 

 real aperture is nearly always rather higher. The focal lengths are 

 exactly as stated. 



Apochromatic Objectives. 



The dry objectives of • 95 aperture and the water-immersions 

 are always provided with correction collars. The divisions on the 



* See supra, lines 8-12. 



