203 



ON DARK-GROUND ILLUMINATiON. 



By Edward M. Nelson, F.E.M.S. 



(^Read March 2Mh, 1911.) 



As this kind of illumination is so much in use at the present 

 time, a few notes on the subject may not be out of place. The 

 illuminant is presumed to be a paraffin lamp with a |-in. wick. 

 Now, the microscope trade differs in a certain way from most 

 others ; any one, for example, can buy an original biscuit, soap, 

 cosmetic, or be made very ill by an original quack remedy ; but 

 one cannot buy a microscope lamp of the original pattern. The 

 one I designed and have continuously used for upwards of thirty 

 years has been altered and " improved " by various people, some 

 among them being novices who have only had a microscope a 

 fortnight. The luminosity of the light given by one of these " im- 

 proved " lamps when measured against mine showed a deficiency 

 of no less than 50 per cent, and some are even worse than that. 

 My lamp owed its origin to my method of microscope work, viz. 

 that of viewing the object in a sharply focused image of the 

 flame. When this was done with an old pattern lamp the flame 

 was not only distorted by the imperfect surface of the glass 

 chimney, but was also disturbed by a reflection from the opposite 

 inside surface. 



A black metal chimney with the 3 x 1-in. slip was the 

 obvious method of removing both these errors. 



One of the early " improvements " was to make the inside of the 

 metal chimney a reflecting surface ; another was to put a window 

 at the opposite side with a coloured glass. By these means one 

 or other of the ends for which this form of chimney was 

 designed was defeated. 



This prologue is my apology for inflicting upon you a second 



