OBJECTIVES AND OCULARS. 
55 
“ residual spectrum,” which residue was previously in¬ 
dicated by the fringes and the blurring mentioned. 
And we have, further, objectives midway in quality and 
price between the old achromatics and the new apo- 
chromatics, the latter being of necessity very expensive 
in comparison with ordinary achromatic lenses. 
In the matter of “ correction,” then, we may take the 
achromatic as the lowest class of objective ; the semi- 
apochromatic as next ; the apochromatic as the finest 
objective at present obtainable. 
Certain facts with regard to the use of ordinary “ un¬ 
corrected ” achromatic objectives for photography have 
lately been brought to light, and these are of such vital 
importance to the photo-micrographer who is restricted 
as to pecuniary outlay, and who requires only good 
definition irrespective of abnormally large aperture, 
that serious attention must be called to these facts. 
The matter seems first to have been mooted by Dr. 
Piffard, of New York, who noticed that with “ uncor¬ 
rected ” objectives, microscopical and photographic, he 
obtained sharp images, provided he used orthochro- 
matic plates and certain “ screens,” or light filters. 
Mr. T. F. Smith claimed to have gone a step further, 
for he stated that the colour-correct plate was enough 
without the screen ; but as he used an oil lamp, he was 
really using a “ screen ” more or less yellow. The 
writer made numerous experiments with a view to test 
and bring together the experiments of these gentlemen, 
with the result that he made the following statement. 
Many, if not all, ordinary achromatic objectives, which 
