November"] 

 1956 J 



The J'iclorian Xatitra/ist 



111 



ON CENTRING MICROSCOPE OBJECTIVES 



By C. S. MlDDLETOX, F.K.M.S., F.R.A.S. 



On reading the article entitled "On Cleaning Microscope Lenses" by H. 

 Snell in Vict. Sat. 72: (August, 1956), I was rather perturbed by some 

 statements in it. While agreeing in general with the method suggested tor 

 cleaning eyepieces, lens tissue or Kleenex tissue is much better than hand- 

 kerchiefs as when new it contains no grit — thus lessening the danger> of 

 scratching the lens. 



My main criticism is directed at the instructions for the cleaning of 

 objectives. Even a low power objective, when new. is perfectly centred, i.e. 

 the front and back components are centred with each other and only under 

 these conditions can the objective perform at its best. When the components 

 of an objective are unscrewed and reassembled without any attempt at 

 centring, they nearly always screw up to a different position, and are 

 therefore out of centre with each other. While this does not make a very 



Centring Process 



big difference to the performance of the lens — the loss of from 5 pet cent 

 to 10 per cent does render a high quality objective equal to a mediocre lens. 

 Should any members of the .Society have followed the advice of the afore- 

 mentioned article, the following apparatus may make it possible for them 

 to re-centre their objectives. 



This centring device, as shown by the illustration, has a mechanical part 

 consisting of a steel ring having the R.M.S. standard thread. This ring is 

 so mounted that it may be centred axially by means of the three centring 

 screws — A on the side and it may be rocked by means of the three screws 

 underneath — B. This is carried on a shaft, which runs in ball bearings and 

 may have a pulley wheel at the bottom as illustrated. This is driven quite 

 slowly, about 120 r.p.m. or less. 



To use this apparatus, the back component of the objective is screwed into 

 the ring. An image of the graticule C in the lamp as illustrated, is focused 

 by means of the lens D on to the surface of the back lens of the objective 

 and the six screws adjusted until the image remains perfectly stationary 

 as the objective is revolved. (See figure.) The second lens of the objective 



