INVERTEBEATA, CRYPTOGAMIA, MICROSCOPY, ETC. 323 



be left in great abundance in a quantity not larger than a table- 

 spoonful. 



Pringsheim's Method of Investigation by concentrated Sunlight.* 

 — We have already given at p. 117 the substance of Dr. Pringsheim's 

 paper on the Function of Chloroi^hyll, so far as regards the results 

 arrived at, but we add a somewhat fuller explanation of what the 

 author terms " a new and peculiar method " of investigation in con- 

 centrated sunliglit. 



He has made use of this method for some years in order to gather 

 experimental knowledge of the relations of light to the absorption of 

 gases by growing plants, and of the part played therein by chloro- 

 phyll. Amid the confusion of contradictory opinions and statements 

 which pervade the literatui'e of the subject, after many vain endea- 

 vours to advance upon the path usually trodden, he turned to the 

 employment of intensified light, hoping thus to be able in a short time 

 unequivocally to observe in the cell, and immediately under the 

 Microscope, the processes called forth in plants by the action of light. 

 The experiments which have hitherto been made have laboured under 

 the serious defect that a too inconsiderable intensity of liglit was em- 

 ployed. This is especially true of those experiments in which it was 

 endeavoured to prove that the diiferent colours of the spectrum act 

 diflerently upon plants. If plants are grown in diftused daylight, or 

 even in direct sunshiue behind coloured screens or coloured glasses or 

 liquids, they evidently grow in relative obscurity in comimrison with 

 their normal condition, even in relation to the colour, the action of 

 which is wished to be investigated ; hence the results thus obtained 

 correspond only to the actions produced in plants by insufficient inten- 

 sities of liglit. Moreover, the function of chlorophyll itself contributes 

 to the weakening of the result. 



So long as only comparatively inconsiderable augmentations of the 

 intensity were employed, no decisive results were obtained. Satisfactory 

 effects were at last attained when organic forms, vegetable and animal 

 cells and tissues, were brouglit into the plane of an image of tlie sun 

 projected in the focus of an achromatic lens of GO nun. diameter. 



The apprehension which perhajis at first arises, that organic 

 structures must under these circumstances be forthwith destroyed by 

 the thermal action of the solar image, is, as a closer consideration and 

 direct experiment show, unfounded. Witli jn-oper precautions, the 

 object can be observed undisturbed for a considerable time in the sun's 

 image, as indeed is approximately sliown by the phenomena in the 

 so-called solar Microscope. In tliis way tlie intiuencc of tlie radiation 

 upon an entire tissue, and ujion each single cell, nay, even upon the 

 different form-constilnvnts of a single cell, can be separately studied, and 

 with a little attention the tlicrmal and photochemical effects of the 

 radiation can be certainly and sliarjdy distinguished. 



Hence this mctliod of microscopical photochemistry (as the author 

 calls it) is pre-eminently adaptoil for investigating whether any, and 

 what, photochemical actions of light take jdace in jjrotoplusm and in 



* 'MH. K. rroiiss. Akiul. Wi.s.s. lU'ilin,' ISTH, ji. W.Vl; sec ' Ann. iiiid Mii" 

 Nnt. IIi.st.,* V. (1880; i>. (ili. 



Y -1 



