330 



Sir C. Wheatstone on a Cause of Error [Apt. 28, 



be further corrected, the component glasses being already closely screwed 

 up together, a further correction can be applied by means of the adjustments 

 of the aplanatic searcher itself, all of which are essentially conjugate with 

 the actions of the objective and the variable positions of its component 

 lenses ; so that if c.v be the traversing movements of the objective lenses, 

 ( > that of the searcher, F the focal distance of the image from the ob- 

 jective when cx vanishes, / the focal distance of the virtual image formed 

 by the facet lenses of the objective, 



*» = /JY 

 a* V / 



The appendix refers to plates illustrating the mechanical arrangements 

 for the discrimination of eidola and true images, and for traversing the 

 lenses of the aplanatic searcher. 



The plates also show the course of the optical pencils, spurious disks of 

 residuary aberration and imperfect definition, as well as some examples of 

 " high-power resolution" of the Podura and Lepisma beading, as well as 

 the amount of amplification obtained by Camera-Lucida outline drawings of 

 a given scale. 



III. " On a Cause of Error in Electroscopic Experiments." 

 By Sir Charles Wheatstone, F.R.S. Received April 26, 1870. 



To arrive at accurate conclusions from the indications of an electroscope 

 or electrometer, it is necessary to be aware of all the sources of error which 

 may occasiou these indications to be misinterpreted. 



In the course of some experiments on electrical conduction and induction 

 which I have recently resumed, I was frequently delayed by what at first 

 appeared to be very puzzling results. Occasionally I found that I could 

 not discharge the electrometer with my finger, or only to a certain degree, 

 and that it was necessary, before commencing another experiment, to place 

 myself in communication with a gas-pipe which entered the room. How 

 I became charged I could not at that time explain ; the following chain of 

 observations and experiments, however, soon led me to the true solution. 



I was sitting at a table not far from the fireplace with the electrometer 

 (one of Peltier's construction) before me, and was engaged in experimenting 

 with disks of various substances. To ensure that the one I had in hand, 

 which was of tortoiseshell, should be perfectly dry, I rose and held it for a 

 minute before the fire ; returning and placing it on the plate of the elec- 

 trometer, I was surprised to find that it had apparently acquired a strong 

 charge, deflecting the index of the electrometer beyond 90°. I found that 

 the same thing took place with every disk I thus presented to the fire, 

 whether of metal or any other substance. My first impression was that 

 the disk had been rendered electrical by heat, though it would have been 

 extraordinary that, if so, such a result had not been observed before ; but 



