232 



THE AMEEIOAN MONTHLY 



[December, 



gard to the spectrometrical constants 

 of the various kinds of glass employed, 

 and the numerous conditions which 

 have to be simultaneously fulfilled. 

 The technical execution is carried out 

 exactly on the data furnished by these 

 calculations, with the strictest check 

 on all the elements in the various 

 stages of manufacture, and without 

 any subsequent empirical touching up. 



In the test given below the objec- 

 tives are given according to their ap- 

 erture. In the second column are 

 the diflbrent focal lengths, while the 

 third column gives the corresponding 

 amplification obtained with the ob- 

 jective (the quotient of the conven- 

 tional distance of distinct vision, 350 

 mm., divided by the focal length of 

 the objective) . 



The objectives are constructed ac- 

 cording to order, either for the Con- 

 tinental length of tube of 160 mm. 

 or for the English of 350 mm. (or 

 10 in.) The three dry objectives, 

 of 6-12 and 34 mm., focal length, 

 are, however, made exclusively for 

 the English tube-length, as these ob- 

 jectives are not adapted for the Con- 

 tinental form. The tube-length is 

 measured from the upper surface of 

 the setting of the objective to the up- 

 per margin of the body-tube on 

 which the eye-piece rests. 



Great care must be taken to pre- 

 serve the correct tube-length, as any 

 deviation materially injures the per- 

 formance of the objectives, particu- 

 larly those for homogeneous immer- 

 sion. 



The settings of all 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 or- 

 dering it is desirable that these three 

 points should be specified, so as to 

 avoid any mistake as to the particular 

 objective required ; (thus : appo- 

 chi'om., 1.30-2.0 mm., short tube). 



The apertures are the guaranteed 

 minimum values ; the real apertm-e 

 is nearl}' always rather higher. The 

 focal lengths are exactly as stated : — 



NuJn' cal Equivalent Objective 

 Ap'rt'e. /ocal Igth. Magnification 

 in nun. for 2jo mm. 



I 0-30 I 

 ^■^>- \ -A 'l-.o 



I -^^ { 



Water immersion 1.25 



H • i '-{ 



hlom. immer. ■; ', 



The dry objectives of 0.95 aper- 

 ture and the water immersions are al- 

 ways provided with correction-col- 

 lars. The divisions on the collar 

 give the thickness of cover-glass in . » 

 hundredths of a millimeter. The cor- 

 rection for the proper thickness of 

 cover must always be carefull}' made 

 when using these objectives, or other- 

 wise there will be a considerable fall- 

 ing oft' in their performance. 



The homogeneous immersion ob- 

 jectives are only supplied in fixed set- 

 tings, as any alteration in the distance 

 of their lenses interferes with the 

 perfection of the correction. Slight 

 variations in the thickness of the 

 covers from the medium value, 0.16 

 mm., for w'hich the objectives are 

 corrected, have no influence on the 

 image, but considerable variations 

 should be compensated for by slightly 

 lengthening the body tube with thin- 

 ner covers and shortening it with 

 thicker ones. 



The slightly thickened cedar-oil 

 (yj d = 1. 5 15) accompan3'ing the ob- 

 jectives (and to be obtained at any 

 subsequent time) should alone be 

 used. Other substances should not 

 be employed unless measurements of 

 the refractive index and dispersion 

 show exact correspondence with it. 

 Mixtures of fennel oil and such like 

 endanger the objective. 



To meet the desire for the highest 

 possible objective magnification the 

 homogeneous immersions are also 

 made with a shorter focal length of 3 

 mm., as well as with one of 3 mm., al- 

 though it must still be regarded as an 

 open question whether any decided ad- 

 vantage can be gained by the former. 

 The impassable barrier to the increase 



