BAIKD &TATLOCK. (LONDON) LTD. 



16 mm. Objective. 



4 mm. Objective. 

 1343 



1.9 mm Objective. 



1343 T Objectives for Bausch & Lomb's Microscopes. 



The tube length for which all of these objectives are computed is 160 mm. (about 6J in.), reckoned from the 

 upper end of the draw-tube to the shoulder of the objective screw. The tube length may be accurately adjusted 

 by means of the draw- tube, which is graduated in single millimetres. 



Cover-Glass Thickness. All dry objectives are corrected for a cover-glass thickness of 0.18 mm., the mean 

 thickness of No. 2 cover-glass, which we have found most practical for general use. For critical work, where an 

 objective is expected to show all its efficiency, measured cover-glasses of 0.18 mm. thickness should always be 

 employed. This is very important as a variation of 0.03 mm. in the thickness of the cover-glass may destroy the 

 spherical correction, and with it the definition of the object. 



The influence of slight differences in the thickness of the cover-glass may be compensated for by increasing 

 the tube length in case of too thin a cover-glass, and shortening it for one too thick. The amount of compensation 

 thus obtainable varies with the E.F. and the N.A. of the objective. In a 4 mm. objective of 0.85 N.A., for instance, 

 an increase in tube length of 30 mm. will balance a decrease in cover-glass thickness of 0.03 mm. 



The performance of homogeneous immersion objectives is quite independent of variations in the thickness 

 of cover-glass, as the refractive index of our immersion liquid (cedar oil) is practically the same as that of the cover- 

 glass. On the other hand, the correct tube length (160 mm.) must be very strictly adhered to, a variation of 5 mm. 

 being sufficient to destroy the perfection of the image. Only the specially prepared immersion oil should be 

 employed. 



Test of High Power Objectives. Unless higher power objectives (from 8 mm. up) are tested under the circum- 

 stances for which they are corrected 160 mm. tube length and 0.18 mm. cover-glass, the best results cannot 

 be expected. This should be borne in mind especially when comparisons are made with objectives of other makes 

 on slides with unknown cover-glass thickness. The finer the corrections and the adjustment of an objective, the 

 more sensitive it is to any change in the conditions mentioned above. 



It may be mentioned here that the better the correction of an objective, the larger a cone of illumination it will 

 stand. 



In the low power objectives particular attention has been paid to securing a large, flat and brilliant field. In 

 the higher powers, while the field is necessarily more restricted, the corrections outside of the optical axis have been 

 carried to a high order. The working distance (measured from the front mount of the objective to the upper 

 surface of the cover-glass) has been kept, in every instance, as long as possible without sacrificing the perfection of 

 the corrections. 



Objectives (see page No. 267). The 48 and 32 mm. objectives are single achromatic lenses, well corrected and 

 giving good illumination. 



The 16 mm. objective of 0.25 N.A. is composed of two doublets. The high character of its correction in the 

 centre of the field, as well as near the margin, makes this lens of great service in low power work. 



In the higher power dry objectives of 8, 4 and 3 mm. focus we use a non-achromatic front lens of nearly hemi- 

 spherical form in combination with two doublets. In the oil immersion the so-called duplex front is employed, 

 consisting of a hemisphere and a meniscus lens, in combination with two correcting doublets. 



The 8 mm. ($ in.) objective of 0.50 N.A. has found great favour in histological investigations, where a high 

 order of correction and great brilliancy, with relatively high magnification, are desirable. Because of its long 

 working distance, it also is eminently useful in blood counting and micro projection work. 



The 4 mm. (J in.) objective is supplied in two types of different N.A. The 0.65 N.A. is distinguished by an 

 extraordinarily long working distance, which enables the objective to focus easily through the thickest cover-glass 

 of the Thoma-Zeiss hsmacytometer. 



The 4 mm. objective of 0.85 N.A., with less working distance and less depth of focus than the 0.65 N.A. type, 

 has as a compensatory advantage the greater resolving power. It is in every sense an optical instrument of the 

 highest quality and is regularly supplied with Bausch & Lomb's outfits. 



Some microscopists prefer to obtain a given magnification with a higher power objective and relatively low 

 power eyepieces, rather than with a low power objective and high power eyepieces. To this end, use a 3 mm. 

 ( J in.) objective of 0.85 N.A., which gives with lower power eyepieces the magnifications otherwise obtainable with 

 the 4 mm. 0.85 N.A. 



The immersion objective now furnished, the 1.9 mm. (^ in.) of 1.30 N.A., is intended for microscope work ot 

 the highest character. Indeed its superior chromatic and spherical corrections entitle it to the appellation of true 

 semi-apochromat, while its free working distance is longer than is usually the case in objectives of this power. 



The full efficiency of an oil immersion objective is only utilized when the substage condenser is also immersed 



For prices see page 267. 





14 -CROSS STREET HATTOTST GARDEN, E- C 



