ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 851 



collar give the thickness of cover-glass in hundredths of a millimetre. 

 The correction for the proper thickness of cover must always he care- 

 fully made when using these objectives, or otherwise there will be a 

 considerable falling-off in their performance. 



The homogeneous-immersion objectives are only supplied in fixed 

 settings as any alteration in the distance of their lenses interferes 

 with the perfection of the correction. Slight variations in the thick- 

 ness of the covers from the medium value (0 s 16 mm.) for which the 

 objectives are corrected, have no influence on the image, but consider- 

 able variations should be compensated for by slightly lengthening the 

 body-tube with thinner covers and shortening it with thicker ones. 



The slightly thickened cedar oil («D = 1*515) accompanying 

 the objectives (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 cor- 

 respondence with it. Mixtures of fennel oil and such like endanger 

 the objectives. 



To meet the desire for the highest possible objective-magnifica- 

 tion, the homogeneous-immersions are also made with a shorter focal 

 length of 2 mm., as well as with one of 3 mm., although it must still 

 be regarded as an open question whether any decided advantage can 

 be gained by the former. The impassable barrier to the increase of 

 useful magnifying power which is fixed by the limit of aperture at 

 present attainable, can already be reached, without loss, by an objec- 

 tive of the focal length of 3 mm., as the latter objective will bear the 

 application of correspondingly higher eye-pieces without any appre- 

 ciable detriment to its performance. 



The objectives (homogeneous-immersion) of 1 * 30 aperture have 

 so great a working distance that they will work through covers more 

 than * 30 mm. in thickness. With an aperture of 1 ■ 40 the working 

 distance is reduced to 0*25 mm. These objectives require very care- 

 ful handling, because in order to obtain the larger aperture the metal 

 setting of the front lens has to be turned extraordinarily thin so that 

 any blow or strong pressure upon the front of the objective is likely 

 to injure it. For both reasons, therefore, the objectives with the 

 slightly lower aperture are undoubtedly more convenient for regular 

 use. The larger aperture will, however, of course allow of a rather 

 higher degree of optical performance being reached. 



No attempt is made to exceed an aperture of 1 * 40, a3 the small 

 percentage of possible increase would render the objectives almost 

 valueless for any scientific investigation. 



With regard to the prices of the objectives, which, especially in 

 the case of the dry series, may appear to be very high in comparison 

 with the usual charges, it must be borne in mind that the apochroma- 

 tics are far more complicated in their construction, and if their special 

 qualities are to be maintained they must be far more difficult to 

 manufacture than the ordinary objectives. Moreover the number of 

 such objectives manufactured must be extremely limited, even with 

 the resources of a large factory. The objectives, however, like all 

 productions of our firm, stand on an absolutely free basis. The glass 



3 K 2 



