20 THE PHYSICAL BASIS OF LIFE 



resolving power of the microscope as limited by the wave- 

 length of light ceases at a magnitude which in round num- 

 bers is not less than 200 submicrons, or somewhere in the 

 neighborhood of /4 25000 incli/^ Such an order of magni- 

 tude seems to be far greater than that of the molecules of 

 proteins and other inorganic substances. Estimates con- 

 cerning the size of such molecules are perhaps not very 

 safe, but it seems to be fairly well established that the 

 molecule of such a protein as casein or haemoglobin is not 

 more than about ^/so of the foregoing figure.'' Even allow- 

 ing for a large error, therefore, an inmiense gap remains 

 between the smallest bodies visil)le with the microscope 

 and the molecules of even the most complex organic sub- 

 stances. For these reasons alone, even had we not the evi- 

 dence of the ultra-microscope, we should be certain that 

 below the horizon of our present high-power microscopes 

 there exists an invisible realm peopled by a multitude of 

 suspended or dispersed particles, and one that is perhaps 

 quite as complex as the visible region of the system with 

 which the cytologist is directly occupied. 



We have now arrived at a borderland, where the cytolo- 

 gist and the colloidal chemist are almost within hailing 

 distance of each other — a region, it nnist be added, where 

 both are treading on dangerous ground. Some of our 

 friends seem disposed to think that the cytologist should 

 halt at the artificial boundary set by the existing limits of 

 microscopical vision and hand over his inquiry to the bio- 

 chemist and biophysicist with a farewell greeting. The 

 cytologist views the matter somewhat differently. Unless 

 he is afflicted with complete paralysis of his cerebral pro- 

 toplasm he can not stop at tlie artificial boundary set up 

 by the existing limits of microscopical vision. He is rudely 

 pushed forward by the impact of a series of stubborn 

 facts with which he must somehow try to reckon. He can 



