GOO PROCEEDINGS OF THE SOCIETY. 



a uniform Microscope and a uniform distance. Was any difference made 

 by variations in the correction ? Mr. Cheshire spoke of parallel rays, 

 and in these there was only one focus. 



Mr. Cheshire, in his reply, said that his apparatus gave simply the 

 distance on the axis between nodal and principal focal points without 

 any reference to the positions of the conjugate points for which the ob- 

 jective was corrected, i.e. for which it satisfied the sine condition. 



Mr. Rheinberg said he would like Mr. Cheshire's opinion on an old 

 " pinhole " method, which he found convenient and very quick for 

 measuring the focal lengths of complicated lens systems, though he had 

 not attempted to apply it to Microscope lenses. A pinhole photograph 

 of, say, 50 cm. focus was taken, once and for all, of a distant view in- 

 cluding two points of reference — for example, two lamp-posts. To 

 measure the focal length of a lens system it was then only necessary to 

 focus the same view by its means on a piece of ground-glass, and measure 

 the distance between the two lamp-posts in the image. The desired 

 focal length then equals : — 



Distance between the lamp-posts in the image x 50 cm. 

 Distance between the lamp-posts in the pinhole photograph. 



Mr. Cheshire pointed out that he (Mr. Rheinberg) was evidently 

 referring to an approximate method of finding the focal length of a 

 photographic lens in which, in effect, the size of the image of a distant 

 object of known angular size is determined. The preliminary pinhole 

 determination gives this angular magnitude once for all so long as the 

 same distance of the object is maintained. 



Mr. T. F. Connolly said that as a visitor to the Society he would 

 like to say he was extremely interested in the simplicity and neatness of 

 Mr. Cheshire's methods. He thought the last instrument described was 

 theoretically perfect, but there was one practical objection to it from 

 the point of view of anyone desiring to determine varied focal lengths. 

 The objective had practically to be mounted as a telescope pivoted over 

 the graduated circle. Dealing with varying focal lengths would mean 

 that a fairly elaborate apparatus to take the objective would be de- 

 manded. He would bke to congratulate the speaker on the simplicity 

 of the apparatus shown. 



In reply to Mr. Connolly's objection, Mr. Cheshire said he agreed 

 with the speaker that the rotating table focometer was not well 

 adapted for determining focal lengths quickly. It furnished rather, in 

 his opinion, a court-of-appeal method. In the collimator method the 

 telecentric disc was preferably adjusted in the principal focal plane of 

 the objective to be tested by using a two-lens afocal system as an eye- 

 piece, or more simply by taking out the draw-tube and focusing the 

 objective directly by looking down the body tube. As a matter of fact, 

 however, the remarkable thing was that in this apparatus, once the 

 scale in the collimator had been adjusted very accurately, it appeared to 

 be almost a matter of indifference where the lens whose focal length 

 was to be tested was placed so long as the highest order of accuracy was 



