achromatic D upwards), with the exception of the homogeneous immersion ob- 

 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 millimetres. 



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

 rule, sufficient for ordinary work to use covers of an estimated thickness, but 

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

 p. 95). 



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. 45. 



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

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

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



Objectives with correction adjustment. The divisions and numbers on 

 the correction collar (bb, Fig. 1), which read by a fixed index, indicate (in hun- 

 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 thick- 

 ness must always be carefully adjusted - - par- 

 ticularly in the case of the apochromatic 

 objectives 6 mm, 4 mm, 3 mm (dry) and 2.5 mm 

 (water immersion) -- otherwise the efficiency 

 of the lenses will be greatly diminished. 



The homogeneous immersion objectives are 



supplied in fixed mounts only, since their 

 efficiency, within rather wide limits, is in- 

 dependent of the thickness of the cover- 

 glass, and also because any alteration in the 

 lens distances would interfere with the perfection 

 of their correction. Considerable variations in 

 the thickness of the cover-glasses are best com- 

 pensated for by slightly lengthening the 

 body-tube in the case of excessively thin 

 cover - glasses and by slightly shortening 

 thick ones. 



Fig. 1. 



Objective fitted with 

 correction-collar. 



The correction - ring bb serves to 

 adjust the distance between the two 

 upper double lenses and the two 

 lower lenses attached to the mount aa. 

 (Full Size.) 



it in that of excessively 



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, 



