262 PROCEEDINGS OF THE SOCIETY. 



that evening, he had been afforded an opportunity of making a rough 

 experiment with it, and so far as he had tried it the results appeared 

 to be strictly exact. One did not always realise the exactness with 

 which lenses were made, and he had been surprised to find with what 

 precision the result was obtained. He should like to ask with what 

 degree of precision it was capable to obtain results by different methods 

 which Prof. Wright had described to them — how in this respect the 

 various methods compared with one another. He had listened with 

 very great interest to this communication, and was very much obliged to 

 Prof. Wright for bringing it before the Society. 



Mr. Beck said he wished to join Mr. Gordon in thanking Prof. 

 Wright for bringing the subject before the Society, and he had no doubt 

 there were many points in it which were entirely new to those present. 

 'There was one suggestion he should like to make, and that was that 

 for the most accurate measurement the micrometer might have a 

 ground-glass surface, because a lens of this kind would be found to have 

 a considerable depth of focus. In a somewhat similar apparatus used 

 for another purpose, the micrometer is engraved on a mother-of-pearl 

 surface. They would, of course, naturally appreciate the anatomical 

 simile which had been applied to the Microscope. He dissented, how- 

 ever, from the conclusion drawn, because microscopists had a way of 

 beheading a Microscope, and of putting the head of one on the body of 

 another, and also of extending the trunk ; it was, therefore, essential that 

 they should obtain the magnifying power of the portions into which they 

 divided it, that is to say, the magnifying power of the complete eye- 

 piece and the magnifying power of the object-glass, because if too much 

 power were put into the eye-piece, and too little into the objective, they 

 would get inferior resolution. The greater part of the work should be 

 done by the objective and not by the eye-piece to obtain the best results, 

 and they must have the means of knowing not only the magnifying 

 powers of the two ends of a system, but also the length of the body. 

 There was no difficulty in giving the standard power to the eye-piece, 

 which always gave the same power under all circumstances, but with 

 regard to the object-glass this was not so easy, because it was giving 

 different magnifying powers according to the length of the body. Any 

 method that tried to give fixed magnifying power for an object-glass 

 led one into a hopeless quagmire when different forms of eye-pieces 

 and tube-lengths were employed. With regard to the instrument which 

 had been described, he could not of course judge fairly of its merits 

 without having used it, but it appeared to l)e most excellent for taking 

 the magnifying power of the complete instrument, though there was 

 likely to be an error in taking the magnifying power of the object-glass 

 separately. It also appeared to have the great merit of being a very 

 rapid and easy means of measuring the actual size of a microscopic 

 object. He felt sure the Fellows present must also have been extremely 

 interested in the diffraction method which had been brought before them 

 by Prof. Wright. 



Prof. Wright expressed his thanks to the Society for the very 

 cordial manner in which his remarks had been received. In reply to 



