442 



the case first considered ; in other words, a microscope-objective 

 would give the anomalous perspective mentioned by Mr. Rhein- 

 berg whenever the limiting aperture was placed behind the 

 upper focal plane. 



There was a very important intermediate case between those 

 illustrated in Figs. 1 and 2 — namely, that represented in Fig. 3, 

 where the iris was placed exactly in the upper focal plane, the 

 consequence being that it could only pass bundles of rays which 

 had been parallel to the optical axis before entering the object- 

 glasSj with the necessary consequence that an object would 



2 



Fig. 3. 



produce a definite and unchangeable size of image at any one 

 distance behind it. Change of focus would only affect the sharp- 

 ness, but not the size of the image. Professor Abbe was the first to 

 point out the value of this arrangement for measuring instruments, 

 and introduced the term " telecentric " for object-glasses with the 

 limiting stop in this particular position. 



Time would not allow him to enter more deeply into this 

 extremely interesting subject, but he might say that it was not 

 difficult to see that a state of affairs as shown in Fig. 2 — which 

 was the usual thing with all except the longest-focus object- 

 glasses — would lead to that peculiar form of distortion which 

 caused a sphere to assume the form of an egg with the pointed 

 end towards the observer, an appearance well known to most 

 workers with binocular microscopes. 



Mr. D. J. Scourfield, F.E.M.S., in introducing the subject of 

 " Mendelism and Microscopy," gave a short account of Gregor 

 Mendel and his work, relating how this Austrian monk, who 

 afterwards became Abbot of Brunn, worked on steadily for some 

 ten years, making experiments upon the hybridisation of peas ; how 

 in 18G5 he published his results, and gave the explanation of them 

 which is known as Mendel's Law of Heredity ; and how this most 

 important paper was completely forgotten until its rediscovery, 



