568 M. A. AINSLIE ON AN ADDITION TO THE OBJECTIVE. 



oiling to the front lens a small piece of cover-glass of the thickness 

 for which the objective is corrected, which will enable it to work 

 exactly as it was intended to work. As far as the aberrations 

 produced are concerned, it is a matter of indifference whether 

 we place the cover-glass next to the object or next to the 

 front lens ; and the advantage of oiling it to the front lens 

 is that there are no reflections introduced to dull the image. 



If the precise change in tube-length necessary for a given 

 change in the thickness of the cover-glass were known in the 

 case of a given objective, there would be no difficulty in calcu- 

 lating the power of the additional lens required to effect the 

 correction ; but, as has been seen, objectives vary so enormously 

 in this respect that it is of little use to give any rules for the 

 purpose. Each objective ought to be fitted with appropriate 

 lenses, just as a defective eye has to be fitted with spectacles 

 Speaking generally, it will be found that with the ordinary 

 l/6th-in., a pair of lenses, convex and concave, if about 10-in. 

 focus, or -f- and 4 diopters, will cover all the ground likely 

 to be required. With lenses of this power, and a range of tube- 

 length from 167 to 259 mm., I find that a Watson l/6th-in., of 

 N.A. 0*74, corrected normally for a tube-length of 200 mm., and 

 a cover-glass 0*18 mm. thick, will give good definition with any 

 thickness of cover-glass from zero to 0*35 mm. ; with a concave 

 lens of 10 diopters, or 4-in. focus, the thickness can be as much 

 as \ mm. Without the additional lens, the variations of thickness 

 of cover-glass which can be allowed for with the above range 

 of draw-tube is from 0*11 to 0'21 mm., so that the introduction 

 of the lenses of -f- an d 4 diopters has more than trebled 

 the range of thicknesses through which the objective will work. 

 This particular objective is rather a favourable example, since 

 its sensitiveness to cover-glass thickness is less than that of 

 many objectives of its power ; but with any objective of this 

 power, and to a somewhat less extent with objectives of higher 

 power, the advantage of this device is evident. With objectives 

 of higher powers, the available range of thicknesses is less, 

 unless the power of the additional lens is raised ; for example, 

 a Leitz No. 7 cannot be made to work on uncovered objects 

 in this way unless with a convex lens of at least 10 diopters 

 power, or 4-in. focus, and even then the result is not nearly 

 so good as in the case of a l/6th-in. This again illustrates the 



