honor of an engagement as contributor to the "Berlin Encyclopædic Lexicon of Medical Sciences." The most important among the numerous articles which he wrote during this period is one on the chemical analysis of the consumption of matter by muscular action; another on animal heat, especially in relation to the question whether the body of an animal throws out the same amount of heat which is produced by the combustion and transformation of its food; and a third, which treats of the development of heat through muscular action.
The great variety of subjects treated in his short publications during these years excludes the hypothesis that there was in his studies a gradual growth toward the discoveries which uphold his worldwide fame, and for which every branch of study related to the physical sciences is so greatly indebted to him. There are, however, impressions which every reader of these papers receives. Helmholtz is nobody's pupil; he stands on the basis of personal observation, and speaks whatever he believes to be true. To show how independently his mind was developed, it may be stated that he could not be induced to attend a single lecture on physical science while a student in Berlin.
In 1847, being consequently only twenty-six years of age, he published his important work, "The Conservation of Force." The discovery of this principle of Nature has been of the greatest moment to the progress of the whole range of physical sciences. This law is, in fact, indispensable to a sound understanding of any and every phenomenon in the animate and inanimate world. And Helmholtz opened his scientific career with a production that would have worthily closed a long life of study and fame.
After the publication of "The Conservation of Force," he was appointed prosector at the Anatomical Institute in Berlin, where he remained about a year. In 1849 he was called to the chair of Physiology at the University of Königsberg. He accepted it, and filled it for a period of nearly six years, in which he made some brilliant discoveries and inventions, which have proved a blessing to thousands of sufferers.
It had been generally held that the time needed for conceiving a thought, and experiencing sensations, could not be measured. Prof. Helmholtz (1850-'51) invented, however, a series of highly-ingenious processes for measuring the duration of any action, however swift, and demonstrated, in a number of papers, that there is a lapse of time before a sensation caused on one end of a nerve is felt at the other. He proved, for example, that, when we touch a thing, it takes a little time before we know that we touched it, and that, however rapid and seemingly instantaneous our actions be, some small period of time must elapse before we can begin to execute the mandates of our will. In 1851 he invented a mirror with which to examine the retina of the eye in living beings, and in the following year he described an
