172 On an Improved Method of 



obtained with the help of the heHostat, except perhaps that in 

 certain cases the prism seemed to offer advantages which will be 

 referred to hereafter. Under these circumstances the heliostat 

 appears desirable for ordinary use, since the solar pencil being 

 thrown in a constant direction, the trouble of adjusting the illumi- 

 nation of a series of objects is considerably diminished, but 1 have 

 convinced myself by trial that equally good pictures can be pro- 

 duced without it, even with very high powers, a circumstance of 

 considerable interest where motives of economy preclude the 

 microscopist from procuring this convenient instrument, 



A few remarks with regard to certain points in the procedure 

 above sketched seem called for. 



First, with regard to the selection of objectives suitable for pho- 

 tographic work of this kind. The power of the objective to be used 

 will depend of coui'se upon the details it is desii'ed to display. I 

 find it best to use the naked objective without eye-piece or amplifier, 

 and not as a rule to fix the sensitive plate more than three or four 

 feet from the stage of the microscope. A ith objective may be 

 conveniently employed to obtain powers of from two to five or six 

 hundred diameters, a yVth for higher powers up to twelve or fifteen 

 hundred diameters. Suitable amplifiers or even eye-pieces may be 

 used in either case, with great increase of the magnifying power, 

 and often with admirable scenic efiect, but there is always a certain 

 loss of definition. Still such amplifications may sometimes be 

 advantageously resorted to, especially in the case of objects which 

 present very minute details ; for in these cases the paper prints will 

 often lose many of the fine details of the negative, and the loss of 

 definition incurred by the amplifier or eye-piece is not unfrequently 

 less than that encountered in attempting to transfer to paper a 

 negative prepared with insufficient magnifying power. Thus far 

 my experience is decidedly in favour of using sufficient power in the 

 first instance, rather than attempting to enlarge negatives taken 

 with less power. 



The objective selected should of course be unexceptional in 

 defining power, and should always be specially corrected for photo- 

 graphy. It has been erroneously stated by Moitessier* that if an 

 ammonio- sulphate or other blue cell be interposed in the solar pencil, 

 all special corrections of the objective may be dispensed with. This 

 proposition, which has been adopted by many other -s^Triters, appears 

 plausible, but a little consideration will show it to be quite erroneous. 



Everyone knows that a good objective must be free from 

 spherical as well as from chromatic aberration. Of course the use 

 of monochromatic light disposes of the chromatic trouble. Not so 

 %vith the spherical aberration. Now this aberration, like the 



* ' I^a Photograpliie Apjiliquec tiux Ivcclicivhca Micrograi)lii(ine«.' Par A. 

 Moitessier, Paris, 18(!0, \). 180 ct seq. 



