236 COBKESPONDENCE. 



cavillers, they might spend their whole time in writing ; and with no 

 effect whatever, except what is expressively called " raising the por- 

 cupine's quills," for microscopists, in general, appear to belong to the 

 genus irritahile. But, here is the letter. 



" No. 170, ErsTox Eoad, London, N.W., Sept. 30, 1873. 



" Deae Sik, — We can answer the question contained in your letter 

 in the last month's ' Monthly ^Microscopical Journal,' respecting the 

 colour you see in the P. formosum when using the immersion arrange- 

 ment. The cause is the ' want of achromatism in the object-glass,' 

 and the reason is that the object-glass was not originally made as an 

 immersion lens. Having to remove the achromatic combination and 

 substitute a single lens, the posterior combinations are not sufficiently 

 over corrected for colour to allow that, consequently you have the red 

 rays predominate. In our immersion lenses now the formula is 

 altered, and they are as achromatic as the dry. We recommended 

 you to have it done for the reason, that you got a longer focus and 

 enabled you to look at your old objects, which you could not do 

 before. The only disadvantage over the present immersions is the 

 little colour ; it makes, as you say, ' a prettier object,' but the picture 

 ig not truthful. 



" Apologizing for troubling you with this, 



" We remain, yours respectfully, 



" Powell and Lealand." 

 From this we learn — 



1st. That if we wish to have a perfect immersion objective, we must 

 have it regularly constructed, througJwiit, as an immersion one : but, 



2nd. If we have one of our old ch-y ones modified into a loet one, we 

 must submit to a (very slight) tint of what the poet Milton calls — 

 *' Celestial rosy red, love's proper hue " ; 



and which really makes, as I have said, " a prettier object." 



After all, however, I hope to live to see the time when the highest 

 attainable perfection of object-glasses shall be effected without the aid 

 of water, or any other fluid ; for though the mode may be ingenious, 

 effective, and mathematical, &c., yet it is a very unpleasant one, to use a 

 thing which will not act (proj)erly) without fii'st having its nose wetted ! 

 In a future letter I purpose stating my own notions of a plan 

 which, possibly, might be a step towards the attainment of this 

 object : meanwhile 



I remain, Sir, yours very respectfully, 



H. U. Janson. 



The Schleiden-Link Question. 



To the Editor of the 'Monthly Microscopical Journal.' 



78, King Williasi Street, E.G., Sept. 11, 1873. 

 Sir, — Although I naturally feel reluctant to reply to an assailant, 

 who thinks fit to conceal his name, yet it seems to me only right to 

 call attention to his mode of attack. 



