PROFESSOR AT HEIDELBERG 185 



carried out, which, by the aid of Maxwell's tops, establish in 

 the case of colours which appear approximately the same to 

 the colour-blind, whether the difference lies in the tone of 

 colour or the degree of saturation. 



Helmholtz's working capacity, however, became gradually 

 exhausted, since the condition of his beloved wife was growing 

 more and more serious ; her relatives took charge of the family, 

 and gave ceaseless attention to the invalid and care to her 

 children. ' It was my privilege/ writes her sister, 'to be with 

 her to the end. She died conscious, in simple strength as she 

 had lived, fearless, with her friend beside her, ever turning 

 towards the highest, on Dec. 28, 1859.' 



Her husband wrote of her : ' I enjoyed the purest and highest 

 happiness that marriage can give one; it was too beautiful 

 for this world/ 



A simple stone marks the grave in Heidelberg churchyard, 

 with the inscription, ' Blessed be the rich seed that Love 

 scatters round it.' 



For many months Helmholtz was totally incapacitated for 

 work by the heavy blow that had befallen him. On April 9, 

 1860, he writes to Donders : ' My warmest thanks to you and 

 yours for your sympathy with my heavy loss. I have been 

 unable to write before, because I have been ill for some time. 

 I had got into a state of nervous irritation from the disturbed 

 nights and agitation of the last few months, so that I could not 

 do any continuous writing without severe headache or attacks 

 of fever. It has been a sad time. In not being able to work, 

 I lost the best means of defence against the feeling that one 

 is alone and has no interests in the world. And I spent two 

 months thus in sleepless nights and weary days. Since the 

 beginning of March I have been able to get a little comfort 

 in work/ 



The fainting fits to which he had been subject of late years 

 increased in consequence of all this trouble. ' Before Whitsun- 

 tide/ he writes on June 27, 'the attacks came on sometimes twice 

 a day; now they are less frequent and severe, so that I can 

 hold up against them if needs be/ But he constrained himself 

 to work, since that alone could fortify him. During March he 

 concluded the paper which he had announced to Ludwig 

 and Thomson the previous summer, in co-operation with 



