The President's Address. By Prof. P. Martin Duncan. 175 



power objectives are good, they are satisfied with, something that 

 will act as a low magnifying power. It does not require much 

 experience to prove the great difierences between the qualities of 

 low powers of the same supposed amplification. A good low-power 

 objective requires more work to be done on it than does a high- 

 power combination, and hence the similarity in cost of a first-rate 

 English low power and a foreign high power. This special work 

 is not always given, and hence the diversity of merit in the lowest 

 powers obtained from different quarters. The absolutely requisite 

 qualities of first-class low-power objectives are a large flat field, 

 achromatism, definition, a good penetrating power, accurate center- 

 ing and as little spherical aberration as is possible. Of the vast 

 number of cheap lenses now produced, it can only be said that 

 it is to be regretted that the public will not consider that they 

 cannot be made good for the money they cost. 



There are very considerable differences in objectives of 1/2 

 in. focus, but it is really a good plan to have one of a considerable 

 aperture and another for ordinary work, with a less aperture and 

 greater penetrating power. Those of the last-mentioned kind are 

 the most valuable with the binocular, after having their brasswork 

 shortened. These lenses and, indeed, all the low powers which are 

 so readily used with the binocular, are often severely tried by it, 

 and their errors become very prominent. Moreover, the indifferent 

 prismatic arrangement of many binoculars is exposed by the 

 objective. 



The lowest powers compete with the hand-lenses, and these are 

 rapidly increasing in excellence. It is remarkable how necessary 

 it is to employ the simple hand-lens after or before submitting small, 

 irregularly shaped, opaque objects, like corals and foraminifera, 

 to the compound Microscope for purposes of illustration. 



Most artists acknowledge that the details of the object are 

 beautifully rendered by the objective and Microscope, but that they 

 only get a true and general idea of it by using the hand-lens. 

 Particular direction of the light, the depth of shadow, and the 

 exaggeration of the details of one part over those of another, 

 characterize the performance of the Microscope armed with a low- 

 power objective ; and a diffused light, less contrasting shadow, 

 and a more symmetrical and general amplification are the gifts of 

 the hand-lens. The Microscope employed with a low-power 

 objective is further removed than the single lens from the eye, as 

 an instrument. These differences are amongst the proofs of the 

 distinction between ordinary and microscopic vision. 



The utility of low powers and their easy manipulation have 

 been greatly increased by numerous appliances, most of which have 

 been carefully considered by this Society. The different kinds of 

 reflectors and condensers of light, and the methods of dark-ground 



