452 SUMMABY OF CURBENT BESEABCHE8 BELATING TO 



Thus, according as the lenses are closed or open {d = 4^4: mm. or 



52 mm.), 



8 = 44 + 24-5 -46-3 = 22-2 



= 52 + 24-5 -46-3 = 30-2. 



Having thus found 8, / is also found, as it is 



24-8 X 48-4 



22-2 

 or 



= 54-1, 



24- 8 X 48-4 _ 3g.„ 

 30-2 



(2) The second step is to find the distance of the posterior focal 

 plane F* of the combination, which being deducted from 10 in. gave 



us A- 



This distance, as the diagram shows, is made up of two quantities, 

 one being the distance of the posterior focal plane F2* of the second 

 lens, which is supposed to be given, and = 48-1 mm., and the other, an 

 unknown quantity, which we will call ^*. This unknown quantity may 

 be determined from the known quantities of /^ and 8 by the formula 



It is therefore 



or 



*" s 



-22^ - ^^^ ^' 

 (48 -4)' 



30'2 



= 77-6, 



according as the lenses are open or closed. 



Adding these values of t,* to 48 • 1 we get the figures given above 

 as the distance of the posterior focal plane from the back lens, i. e. 

 153-6 or 125-7. 



The focal length of the objective and the distance of its posterior 

 focal plane are thus very readily found, without elaborate calculations, 

 by simply knowing the focal lengths and the position of the focal 

 planes of the separate lenses, data which can be obtained very simply 

 and without the necessity of knowing anything about the formulse on 

 which the objective is constructed or the refractive index of the glass 

 of which its lenses are made. We hope, as we have said, to return to 

 this subject hereafter and in more detail. 



Queen's Spot-lens Mounting. f — In order to overcome as far as 

 possible the difficulty J. W. Queen and Co. have felt in fitting the 

 spot-lens to instruments of various patterns (some with movable sub- 

 stage and some with fixed tube, the latter at varying distances from 

 the upper surface of the stage), they have devised the following 

 mount : — 



The tube A (figs. 61 and 62) is made of standard size to fit the 



t Micr. Bulletin, i. (1884) p. 11 (3 figs.). 



