244 president's address. 



other medium ; and it is to assist those in this plight that the 

 following hints have been written. 



The first difficulty, as we have already pointed out, is that of 

 getting sufficient contrast between the object and its background, 

 but that is not, unfortunately, the only one with which we have 

 to contend. It is by no means easy when high magnifications 

 are employed to obtain really crisp photographs perfectly free 

 from fog. The fault seems to be this : the small markings 

 whether clots or pearls, may be perfectly well defined and per- 

 fectly crisp, but they appear as if they are mounted in fog. The 

 appearance presented is often put down to over-exposure, but if 

 a shorter one be tried no benefit accrues ; the only change in 

 appearance is that the photograph seems much fainter. To what 

 then is this difficulty due % On not a few occasions the optician 

 is blamed for omitting to carry his corrections sufficiently far, 

 whilst the emulsion comes in for its share of abuse, the bogey 

 of orthochromatism being brought in somehow or other. But in 

 truth the trouble may not be caused by any of these supposed 

 faults, but may be traced to optical difficulties, about which we 

 shall now very briefly speak. 



The laws of optics tell us that, owing to the finite wave- 

 length of light, the image of a point cannot possibly be represented 

 by any lens or combination of lenses as another point, but must 

 be shown as a disc of more or less sensible diameter. Now this 

 circle of confusion varies very largely with the magnification ; 

 hence, when great amplification obtains, this circle is correspond- 

 ingly increased in diameter. Hence the dot or pearl appears not 

 as a circumscribed dot, neat and crisply defined, but as one sur- 

 rounded by what for our present purpose we may call " fog." It 

 is easy to understand that the halo of fog around one dot joins 

 up with the halo of its neighbouring dot, so that in the case of a 

 row the Avhole series appear as floating in haze, the very appear- 

 ance of which we have been complaining. Having ascertained 

 the cause, what can we do to remove it ? Seeing the optician 

 can do nothing, save to tell us not to magnify so much — a pro- 

 ceeding we cannot avoid, because the objects are so exceedingly 

 .small — we should be at a loss to know what to do did not the 

 photographer step in at this juncture to offer his assistance. 

 Seeing that there may be many of our members who have not 

 paid much attention to the scientific side of this subject, I must 



