PRACTICAL PHOTO-MICEOaEAPHY. 89 



of Dr. E. C. Bousfield already alluded to (p. 14), as his 

 table is copyright we do not, even with his permission, pro- 

 pose to use it here. 



The factors on which exposure mainly depends, excluding 

 from our consideration color, are 



1. Illumination. 



2. Magnification. 



1. Illumination. We can no more lay down rules with re- 

 gard to this than with regard to color. An oil lamp is the 

 weakest light generally used, the lime light next, magnesium 

 next, the electric light next, sunlight the most powerful. 

 But all depends on how they are used ; lime light properly 

 burning and properly used may be much more active 

 on our plate than diffused daylight. As we propose to con- 

 sider here artificial light only, and specially the lights we 

 know best, oil and lime, we need only say that the lime light 

 may be from 10 to 50 times as powerful and as actinic as the 

 best oil lamp. 



The effect of the condenser properly used is astounding to 

 the beginner, the bull's-eye sometimes increases and sometimes 

 diminishes the force of the light. 



The beginner after producing a negative will find it very 

 difficult, even with the instructions to follow on later pages, 

 to form a correct opinion as to whether his negative is over, 

 under, or properly exposed. His best plan is probably to 

 show his negative to some experienced photographer who will 

 be able to give a certain amount of help as to the steps to be 

 taken in future. But even the experienced photographer, 

 and in some cases even the experienced photo-micrographer, 

 will be at a loss to decide whether a negative is over or under 

 exposed. In this case the only thing to be done is if the 

 prints are not satisfactory to try a longer exposure and a 

 shorter one. Exposures should be varied by geometrical 

 rather than arithmetical progression ; that is, if, for example, 

 an exposure of 40 seconds is found to be wrong, it is well to 

 try 20 or 80, or even 10 or 100 seconds rather than 35 or 50. 



A weak background (i. e., a ground grey in the negative and 

 dirty, white in the print) is a sure sign of one of three faults : 



