OBJECT GLASSES AND EYEPIECES 81 



from the object glass is not coming to the same point, the illu- 

 mination will not be equally distributed at the two positions 

 (A and B), It is important to select a very small point for the 

 purpose, because many of the objects seen through the micro- 

 scope are partially or completely transparent, and often globular, 

 and act as small lenses themselves, which interferes with the 

 phenomenon unless they are extremely minute. With trans- 

 mitted light on a bright field the same plan may be adopted, 

 but it requires much more careful observation because the image 

 of a fine speck of dust when out of focus appears as a faint patch 

 on a bright field, and is not so easily observed as a bright 

 patch on a dark background. High- power eyepieces should 

 always be used for making these observations. By the examina- 

 tion of insects' scales and diatoms much greater accuracy can be 

 obtained in this adjustment, but the explanation of their use 

 requires further discussion of the theory of the microscope 

 and is not attempted in this book. 



All high- power dry object glasses are made for use with a 

 cover glass -006 inch (-15 mm.) or -007 inch (-17 mm.) thick 

 unless specially ordered to be made for use without cover 

 glass for polished metal specimens. The l/6th-inch (4-mm.) is correction 

 sometimes made with a correction collar, which is an adjustment ^^^- 

 which, by altering the distance between the component lenses, 

 enables the object glass to be corrected for any thickness of cover 

 glass between and -01 inch (-25 mm.). Such a lens in the hands 

 of a beginner should always be used with its correction collar 

 set at about -007 inch (-17 mm.) unless no cover glass is being 

 used. It may be a positive disadvantage to use such an object 

 glass unless the microscopist is practised in the method of making 

 the adjustment. 



The colour correction of an object glass is referred to 

 later, but it should be remembered that an achromatic object 

 glass can sometimes be slightly improved by the use of a 

 colour screen, as such lenses always give a slight indication 

 of faint colour. The faint colour efiects often seen in trans- 

 parent objects are, however, frequently due to the objects acting 

 as small uncorrected lenses themselves, or to diffraction effects. 



The flatness of field of a microscope depends on the object 

 glass and the eyepiece combined. The edge is generally not 

 in focus at exactly the same position as the centre of the field. 

 Such a defect cannot be entirely cured in the best lenses, 

 because to do so would sacrifice the finest definition in the centre ; 

 consequently, for the most exact examination the object must 

 always be brought near to the centre of the field. With low- 

 power lenses the defect is not so apparent, but with high powers 

 there is always a marked superiority in the performance near 



the centre. 



The following table gives the approximate sizes of the field 



