172 GENERAL CONDITIONS OF SCIENTIFIC RESEARCH. 



perceived. In consequence of this, in many cases, several 

 persons have discovered thesame truth at about the same 

 time, having been led to it by similar circumstances, for 

 instance, the discovery of oxygen by Priestley and Scheele, 

 and of the perturbations of the planet Uranus by Adams 

 and Le Verrier ; of polarisation of light in liquids, by 

 Seebeck and Biot ; of refraction of light by Gregory and 

 Willebrod Snell ; of the volumetric composition of am- 

 monia gas by Berth ollet and Dr. Austin ; of the Leyden 

 jar by Muschenbrock and Kleist; of the metal Thal- 

 lium by Crookes and Lamy ; &c. In exemplification 

 of this great general truth it has been stated that, had 

 Newton never lived, the great law of gravitation would 

 have been discovered at a period not much later than it 

 was. It is evident also that experiments requiring parti- 

 cular substances or conditions could not have been made 

 before those substances or conditions were found. For 

 example, those which could only be made by the assistance 

 of gutta-percha or voltaic electricity could not have been 

 made before these agents were known.. 



The invention of the telescope was the starting-point 

 of great astronomical discovery. ' In June 1609, it was 

 rumoured in Venice that an artificer in Flanders had 

 presented Count Maurice of Nassau an eye-glass so 

 cunningly contrived that it made objects far off appear as 

 if they were close at hand. When Gralileo heard this, he 

 immediately returned to Padua, and, after having thought 

 over the matter for a day and a night, he set to work to 

 make his telescope. When it was known in Venice that 

 he had succeeded in constructing the enigmatical machine, 

 he was invited' by the Venetian Republic to present them 

 with his telescope ; he complied with this request on the 

 23rd of August, 1609, and dedicated it to the Doge.' 

 ' Then the Senate, as a mark of appreciation, by a decree 



