IO 



jectives, that thickness of the cover-glass which gives the most perfect correction 

 is indicated on the side of the mount by small figures representing milli- 

 metres. 



With those objectives, which we supply in fixed mounts only, it is, as a 

 rule, sufficient for ordinary work to use cover-glasses of an estimated thickness, but 

 for very exacting tasks we recommend the use of a cover-glass gauge (see p. 79). 



A simple method of fairly accurately determining the thickness of cover- 

 glass requisite when using one of our large or medium stands is explained on 

 p. 42. 



Jf it be desired that an objective should be adjusted for a 

 cover-glass differing front the usual thickness, or for use without 

 cover-glass, this requirement should be specified in the order. 



Objectives with correction adjustment. Fig. 2. The divisions and numbers 

 on the correction collar (bb) , read off by a fixed index , indicate (in h u n - 

 dredths of a millimetre) that thickness of cover-glass which gives 

 the best correction corresponding to the position 

 of the collar. 



The correction for cover-glass 

 thickness must always be carefully adjusted 

 - particularly in the case of the apo- 

 chromatic objectives 6 mm, 4 mm, 3 mm 

 (dry) and 2.5 mm (water immersion) 

 otherwise the efficiency of the lenses will be 

 greatly impaired. 



The homogeneous immersion objectives 



are supplied in fixed mounts] only, since 

 their efficiency, within rather wide 

 limits, is independent of the thickness 

 of the cover-gla|ss, and also because any 

 alteration in the lens distances would interfere 

 with the perfection of their correction. Consid- 

 erable variations in the thickness of the cover- 

 glasses are best compensated for 



by slightly lengthening the body-tube in exceedingly thin cover-glasses, 

 shortening excessively thick ., 



The immersion fluid which we recommend for homogeneous objectives is 

 slightly thickened Cedar-wood Oil (from Juniperus virginiana) as used by 

 us from the first. We. expressly desire to caution against the use 

 of immersion fluids derived from other sources, with our objec- 

 tives, as the use of fluids of irregular refractive and dispersive 

 power may entail considerable impairment of the optical perform- 

 ances of the objectives. 



Fig. 2. 



Objective fitted with 

 correction-collar. 



The correction-iing bb serves to adjust 



the distance between the two upper 



double lenses and the two lower lenses 



attached to the mount aa. 



(Full Size.) 



