QDEKETT MICROSCOPICAL CLUB. 657 



was mentioned that 40 slides illustrative of Palaeozoic Botany 

 had been purchased. 



Mr. Ainslie, R.N., then introduced the following paper, en- 

 titled k " An Addition to an Objective " : Few microscopists 

 who have made much use of high-power dry objectives have 

 failed to realise the connection between the tube-length and the 

 thickness of the cover-glass if good definition is to be obtained. 

 This is, indeed, mentioned in the textbooks, but not, as a rule, 

 at very great length. Little is said, for instance, as to the 

 amount of alteration required in any given case. The sensitive- 

 ness of objectives varies enormously ; to a certain extent with 

 the formula on which the objective is constructed, but more 

 especially with the power. As an example, a 1/2-in. of high aper- 

 ture, such as the Holos or the Zeiss Apochromat, requires 

 a change of one or two millimetres only in the tube-length to 

 compensate for a change of 0*01 mm. in the thickness of the 

 cover-glass ; for l/6th, the figure is from 9 to 13, while for a 

 1/8 th, such as the Leitz No. 7, the figure is as much as 20 or 21. 

 Water-immersions are also subject to this sensitiveness, though 

 to a smaller extent, the figure in the case of a Zeiss " G " being 

 9'2. This feature is more important than is often realised, and 

 the difficulty caused thereby is enhanced by the extremely small 

 range of draw-tube in the average Continental stand, and un- 

 fortunately, in many stands of English make. The present 

 paper is an attempt to find a way out of the difficulty, and the 

 device suggested should be useful when the range of draw-tube is 

 insufficient, especially when the higher powers are in use. Many 

 years ago the celebrated Van Heurck used what he called a 

 "transformer" as a means of making a long- tube objective 

 work on a short tube, and vice versa. This consisted of a con- 

 vex or concave lens of low power, fitted above the objective, 

 which, it will be readily understood, affords a means of altering 

 the actual plane in which the image is formed (without affecting 

 the action of the objective), should it happen that the cover- 

 glass is of such thickness to require, for satisfactory definition, 

 a tube-length which would bring the image beyond the limits 

 of the draw- tube. With the high-power dry objectives in 

 common use, such as the average l/6th, the power of the addi- 

 tional lens required to effect the compensation for a considerable 

 change of cover- thickness is not excessive ; a pair of lenses, 



Jourx. Q. M. C, Series II. No. 77. 37 



