1897] MICROSCOPICAL JOURNAL. 355 



it is well to have a coil giving a greater length of spark, 

 say 6 inches. The tube is much more brilliantly illumin- 

 ated with such a coil, exposure is shortened and deep 

 structures more easily penetrated. There is another rea- 

 son also why so large a coil as a 6 inch should be taken. 

 With use the vacuum of a tube becomes higher and is con- 

 sequently more diflScult to excite. Warming with a spirit 

 lamp will reduce the vacuum but it is not nearly so satis- 

 factory as being able to excite the tube direct from the 

 coil. 



The Tube : — More failures in working have been due 

 to defective tubes than to anything else, in fact a large 

 majority of the tubes that have been sent out have been 

 absolutely worthless. It is unwise to buy any tube with- 

 out a guarantee of its suitability and perfection in work- 

 ing and where such a guarantee is obtained the price is 

 usually somewhat high. Still it is better to pay a fair 

 price for a good article than to have several unsatisfac- 

 tory tubes at a low price. 



As tubes are somewhat liable to damage it is well to be 

 provided with two or three. No absolute statement can 

 be made as to the length of life of a tube. The writer has 

 one in use which has been constantly employed for the 

 past three months and is as good as ever, while others 

 have sometimes failed in some particular after a very 

 short period of use. We have experimented with tubes 

 by all makers and have spared no expense in having the 

 latest patterns as they have been issued, but in our hands 

 the focus tube as manufactured by W. Watson & Sons 

 surpasses every other kind both for the fluorescent screen 

 and for radiographic effects. 



There is no doubt that the whole process is in its 

 infancy and time alone will show in which direction 

 further successful progress in the methods will be made. 

 Supplementary apparatus will also appear to augment its 

 usefulness. 



