QUEKETT raCEOSCOPICAL CLUB. 49 



equal to that of oil of cloves. It is easily soluble in ordinary 

 alcohol, amylic alcohol, ether, and by heating in fixed oils; it 

 is partially soluble in benzol, petroleum spirit, chloroform, and 

 turpentine. Mr, Wall is exhibited some photographs to show 

 the increased visibility obtained in some cases by using this 

 medium instead of ordinary balsam. He also showed a photo- 

 graph taken by dark- ground illumination to prove that the 

 medium was homogeneous, and stated also that it showed no 

 tendency to crystallise nor did the castor oil show any ten- 

 dency to separate. A good clearing and dehydrating agent is 

 made by dissolving phenol crystals in an equal quantity of 

 amylic alcohol. This mixture has not the hardening and shrink- 

 ing effects of clove oil or turpentine. Preparations may be 

 transferred from it directly to the mountant. 



After a few questions had been asked a hearty vote of thanks 

 was passed to Mr. Wallis for his useful communication. 



Mr. Morley Jones exhibited five slides of the heads of male 

 gnats, and described the method of preparing and mounting 

 them. 



Mr. Grundy read a note by Mr. E. M. Nelson on the Optical 

 Index. The Optical Index indicates the ratio of aperture to 

 power, and is obtained by the formula : 



1,000 X N.A. 

 initial magnifying power 



or, which is the same thing, 100 x N.A. x focus in inches. 



The following simple experiment should be performed by all 

 those microscopists who take a scientific interest in the use of 

 their instrument. It is well to remember that there is a vast 

 interval between seeing a picture and reading about one. The 

 only extra apparatus required is a visiting card and a sharp 

 penknife. Place a suitable low-power test object upon the 

 microscope stage (a blowfly's tongue would do very well), put 

 a 2-inch objective on the nosepiece, and use a C eyepiece (x 10). 

 Examine the image with attention ; the optical index of the 

 objective will be somewhere about 20 to 24. Now cut a stop 

 in the card, with a central circular hole 6 mm. in diameter, 

 place it at the back of the objective, and particularly note the 

 difierence in the im^age. The optical index will now be reduced 

 to 12. (Of course a 3/4 cone must be used in eachci.33.) 



JouRN. Q. M. C, Series II.— No. 84. 4 



