288 HOW TO WORK 



lens as before mentioned gives an excellent quality of light, but less 

 intense than Kelner's eye-piece. 



In using these condensers, it is most important to employ the 

 direct light from the lamp. The microscope and lamp are to be 

 arranged as represented in pi. LVI, fig. 344. I cannot explain why 

 the illumination should be so much better than when the mirror is 

 employed, but I am sure that the quality of light produced is much 

 more favourable for minute observation. 



I have tried both the lime and magnesium lights, but they are not 

 suitable for microscopical observation, the glare being too great, and 

 the arrangements necessary inconvenient and troublesome, while 

 paraffin, which can now be obtained everywhere without any difficulty, 

 gives most satisfactory results at perhaps T 1 - ff th of the cost. 



My friend, Mr. W. E. Kilburn has much increased the illumi- 

 nating power of the paraffin lamp by causing a stream of oxygen gas 

 to play around it. The gas is contained in a small bag which is 

 placed under a weighted board. A piece of fine India-rubber tube 

 connects the gas bag with a small pipe by which it is conducted just 

 outside the wick of the lamp. 



362. Method of Increasing the Size of the Image without Altering 

 the Object-Glass. Supposing the limits of magnifying power of the 

 object-glass to have been reached, there are yet methods by which 

 the dimensions of the image may be greatly increased. The eye- 

 piece may be changed for a deeper one, or the distance between the 

 object-glass and eye-piece may be increased. In practice, I have 

 found that the latter plan is so much more advantageous that I now 

 never use a deep eye-piece. 



The -sV objective being applied, when the tube is increased in 

 length, so that from the lowest glass of the object-glass to the eye- 

 glass of eye-piece, the distance measures 24 inches, the magnifying 

 power corresponds to upwards of 10,000 diameters : when the length 

 is 20 inches to about 6,000 : 15 inches to about 2,600 : n inches 

 to about i, 800. When the tube is thus increased in length, there is 

 often some reflection from its interior which renders the image indis- 

 tinct, an inconvenience which may be remedied either by increasing 

 the diameter of the microscope tube to about z\ inches, or by lining 

 the ordinary tube with black velvet. 



:;;:;. Of Drawing Objects magnified with very High Powers. 

 In delineating the appearances observed, I never represent a struc- 

 ture more highly magnified than is necessary to bring out the points, 

 but I have found that as I improved my method of preparation, 

 p. 290, I desired higher magnifying powers, and I am quite certain 

 that great advantage will be reaped when powers far higher than 



