230 HOW TO WORK 



be brought under the notice of some of those who alone have power 

 to forward or obstruct scientific progress in the departments under 

 Government control.* 



" With low powers no serious obstacle was encountered in obtain- 

 ing excellent photographs of properly selected preparations. The 

 higher powers offered difficulties most of which however have been 

 overcome. In experimenting, with the higher powers, the lined dia- 

 tomacse were selected as test objects on account of their definite 

 and well-known structure. With these the utmost success has been 

 realised. A photograph of Gyrosigma angulatum (Navicula angulata) 

 has been obtained, for example, magnified about 7,000 diameters in 

 which the hexagons appear of the same size and nearly as distinct as 

 in the cut, which was made by transferring to wood a tracing from the 

 original photograph. In fact, any of the markings on the diatoms 

 that are visible with the microscope can be photographed with the 

 utmost clearness and ease, and the time has arrived when the inabi- 

 lity to photograph alleged markings will throw doubts on the correct- 

 ness of the observers who have supposed they saw them. The plan 

 employed in the photographic work hitherto executed with high 

 powers is as follows : The direct rays of the sun reflected in a con- 

 stant direction from the mirror of a Silbermann's heliostat (loaned for 

 the purpose by the Coast Survey), are condensed by a large lens 

 upon the plane mirror of the microscope, whence they are reflected 

 through the achromatic condenser in the usual way. Before reaching 

 the achromatic condenser, however, the rays pass through a cell con- 

 taining a solution of the ammonio-sulphate of copper of sufficient 

 density to absorb nearly all the rays except those at the violet end 

 of the spectrum. The light used, therefore, is essentially monochro- 

 matic, and contains, with enough illumination for agreeable vision, 

 the greater part of the actinic force of the sun's rays. The heating 

 rays being chiefly at the other extremity of the spectrum are of course 

 excluded and great actinic force is obtained, therefore, without any 

 danger to the preparations, or the balsam cementing of the object- 

 glasses. The object-glass employed in the photograph of Gyrosigma 



* I believe that it would be most difficult, if not actually impossible for 

 our Government at this time to issue a report of the character of that from 

 which the extract is taken, supposing that the actual work had been done by 

 private persons and placed at the disposal of the State. The paper of our Blue 

 Books is too coarse, and the printing too rough for scientific memoirs. Let the 

 reader, for instance, compare the plates accompanying my report on the Cattle 

 Plague, which were printed by Government, with those in the present work. The 

 contrast between the text of Government and private works is still more 

 striking. 



