274 PROCEEDINGS OE THE 



races or slight variations of a single species. The genus Sarcoptes 

 also consists of a single species. It is, Itowever, very difficult to 

 transfer the mites from one host to another. 



The President remarked on the difficulty of transferring fungal 

 parasites from one host to another. There were fungal races 

 peculiar to different hosts, although apparently they were 

 morphologically identical. A vote of thanks was accorded to 

 Mr. Hirst for his valuable contribution. 



Dr. W. A. M. Smart then exhibited and described a simple 

 apparatus for finding the refractive indices of oils used in medicine. 

 Briefly, it consists of a biconvex lens laid on a plane mirror. A 

 short pin is held at such a distance above it that image and object 

 coincide. The oil to be tested is run in between the lens and the 

 mirror. From a mathematical calculation the refractive index 

 is obtained. It was shown that the result obtained is reliable to 

 three places of decimals — sufficiently accurate for the purpose 

 for which the apparatus is designed, viz. to test the purity of 

 the oils used in pharmacy. Some suggestions were made by 

 Commander Ainslie that in such a case use might be made of 

 Newton's rings for calculating the refractive index. The thanks 

 of the meeting were accorded to Dr. Smart for his exhibit. 



The President then called upon Mr. A. A. C. Eliot Merlin to 

 read his paper on " Some Facts and Fallacies of Practical Micro- 

 scopy." Mr. Merlin said that his aim in reading the paper was to 

 help beginners to obtain trustworthy microscopical images, by 

 which the true structure of minute objects is revealed and the 

 dangers of erroneous interpretation minimised. In his early 

 days he was initiated into the use of the old high-power Seibert 

 water-immersion lenses which he saw used mostly on diatoms with 

 the " white-cloud" illuminator, or a quarter-inch objective used out 

 of focus as a substage condenser. Frequently these were dis- 

 pensed with and the concave mirror used alone. The object was 

 to get sufficient light by any and every means. The books at the 

 time gave no practical help. In 1888 he obtained his first oil- 

 immersion objective of N.A. 1-25 ; this he used on diatoms with 

 an Abbe illuminator and moderate stop, and obtained fairly 

 good views, but the optician who supplied the apparatus advised 

 him to use the smallest stop in his condenser in accordance with 

 the teaching of Abbe. At that time it was considered unthinkable 

 to dispute any of Abbe's inferences from his diffraction theory. 



