OBJECT GLASSES AND EYEPIECES 79 



a pair of conjugate foci. The important characteristic of an conjugate 

 object glass is that it can only be absolutely correct for one pair i"*s«8- 

 of conjugate foci which, as applied to the use of the microscope, 

 means that at one length of drawtube (160 mm.) the image will 

 be clearer than in any other position. It is a point that is 

 sometimes considerably exaggerated by writers on the microscope, 

 who give the impression that if the wrong length of drawtube is 

 used, the object glass is almost as bad as an uncorrected lens. 

 ^The truth is that, especially with high-power lenses used with high 

 eyepieces, for examining the finest details it becomes a factor of 

 importance; but with moderate power eyepieces for general 

 observation, a considerable variation of the length of tube makes Tube length 

 no noticeable deterioration in the sharpness of the picture and™™""""" 

 forms an exceedingly useful means of altering the magnifying 

 power. Low-power lenses, on account of their smaller aperture, 

 are much less sensitive to a change in the length of the drawtube, 

 and when using the l|-inch (4:0-mm. and 32-mm.), 2/3-inch 

 (16-mm.), or even 1/3-inch (8-nma.) object glasses with eyepieces 

 not higher in power than X 10, a variation of 30 or 40 mm. in 

 the drawtube is difficult to notice. With 1/6-inch (4-mm.), 

 1/8-inch (3-mm.), 1/12-inch (2-mm.) object glasses, it is best 

 to use the standard 160-mm. tube length, and if a 1/3-inch (8-mm.) 

 is being used with high eyepiece, the drawtube should be set 

 at its correct length. 



The other important factor in the best performance of an Thioknesa of 

 object glass is the thickness of the cover glass which is between ""^^ ^^^^' 

 the object and the front lens. With high-power lenses, the 

 thickness of this cover glass has a far greater effect on the quality 

 of the image than the length of tube. 



If a cover glass be used which is incorrect it can be corrected correction of 

 to some extent by making an alteration in the length of tube, ^^|^^ 

 and the following table gives the approximate amount of altera- drawtube. 

 tion in the tube length required to correct a variation in the 

 thickness of the cover glass with different powers. An oil- 

 immersion lens is not subject to this variation because it has 

 nothing but glass, or its optical equivalent, between the object 

 and the front lens. If the cover glass is thicker, the cedar-wood 

 oil is proportionately thinner. For this reason the table only 

 refers to dry lenses. It will only enable corrections to be made 

 for cover glasses which vary- between certain limits. For in- 

 stance, a 1/6-inch cannot be corrected for an uncovered object 

 by its means. The low-power lenses are so insensitive to small 

 amounts of variation that the table is chiefly of use in indicating , 

 that the thickness of cover glass is unimportant except when 

 working through thick troughs of water. It should be remembered 

 that water has only about two-thirds the effect of glass, and, 

 therefore, where a trough is used, one-third may be added 

 to the figures given. 



