QUEKETT MICROSCOPICAL CLUB. 99 



difficult to restrict tlie illumination to tliat portion of the 

 field corresponding to the photographic plate. The second 

 method does not possess these disadvantages, for the 

 illumination of the field is uniform, and may be restricted 

 by means of a diaphragm. But, as Ainslie points out, it 

 has the disadvantage that the aperture of the substage con- 

 denser is usually more brilliantly lit at its centre than at its 

 edges, so that with a solid cone coarse details, since they do not 

 require rays of such great N.A., are represented more brilliantly 

 than fine details. This difficulty may be almost eliminated, 

 Dr. Hartridge said, by putting a slip of finely-ground glass 

 between the iris and the bull's-eye ; but some light is lost by 

 doing this. The third method has the advantage of sim- 

 plicity, but Dr. Hartridge thought that as compared with the 

 other two the image was a shade more milky owing to the 

 presence of scattered light. 



Dr. Hartridge has applied to his microscope a fourth 

 method of illumination which is a combination of the second 

 and third. An auxiliary lens is mounted below the iris of the 

 immersion condenser, so that the source of light may be placed 

 at IJ in. away without spherical aberration. At this point a 

 centring iris diaphragm is attached to the dovetail slide in place 

 of the mirror, and below it is a slip of flashed opal glass illuminated 

 from behind by a small Osram lamp. The light-source is there- 

 fore attached to the microscope, and with the iris of the 

 illuminant open a large uniform light is available for low powers, 

 whereas by closing it the illuminated part of the field may be 

 restricted for high-power work. The intensity may if necessary 

 be controlled by means of a simple adjustable resistance. 



Dr. Hartridge received a very hearty vote of thanks for his 

 valuable communication. 



At the 544th Ordinary Meeting of the Club, held on June 10th, 

 1919, the President, Dr. A. B. Eendle, F.E.S., in the chair, the 

 minutes of the meeting held on May 13th were read and confirmed. 



Messrs. Leonard F. Lawson and H. Alfred Dodd were balloted 

 for and duly elected members of the Club ; three nominations were 

 read for the first time. 



The President said that the difficult question of the Club's 



