MEDIUM-POWER PHOTO-MICROGRAPHY 91 



little need be said. Achromatic objectives require one sort, the ordinary Huygheniau, 

 and apochromatics another which are called " Compensating " because they complete 

 the colour correction of the objectives. For low-powers the best form for either work 

 IS what is called " a projection eye piece," but for magnifications that reach to thousands 

 of diameters in the case of apochromats the high-power compensation forms must be 

 employed. They can be obtained as high initially as 27. One point the photo- 

 microgranher should never omit to guard himself against, and that is, the accumula- 

 tion of dust in the oculars : they not only lessen the amount of light but seriously 

 spoil the final performance. There is a certain class of dirb which settles on the 

 lenses not very evident until a photograph is taken, when it will appear as a kind 

 of ill-defined dot with a bubble-like periphery. The lenses often require to be cleaned 

 with spirits of wine to remove the source of trouble and a ivell-ivashed chamois 

 leather kept in a stoppered bottle should be at hand for this purpose. 



