Paper read at the meeting ofthe British Association for the Advancement ofscience, a t Cambridge on August 19,1938) .During recent yccirs t h pliotoclcctric cell lins nttiiIiictl i t t i cstriiortlitiririly liigli stiilidiircl of cfiicicncy, pertiiittiiig its wide :itloption in prncticnl lifc, itidustry, niitl research. 111 cliciiiical, Iioclicmiciil, iiiitl mcdicnl brnnchcs of scierice the pltotoclcctric principle lins been iipplictl to wlint iiiiiy be tennet1 stntionary obscrvat.ioiis, that is, observiltions of tlie degree of triiiismittetl light by ccrhiii specimens. Tliis siliiplc detcriiiiiiatioii of tlic tlciisity of lion-colourcd iiiid colourcd specimens (deiisitonietry, photometry, pliotoclcctric colorinict,ry) lins the wltolc iLI)1)nriittts iirc given i n Pig. 1, in soincwhnt. sinq!lilietl fortii.
Tlic Pliotclgrnpli consists of iL registering unit iiud 41 rccorcling unit. The registering system coiisists of :I wttcr. h t l i (WB, Fig. 3 ) betweerr i~ coiistaiit source of' light (U) itlid R photrotiic, photoelectric ccll (Ph) which niny be coiiiicctcd with eitlicr tlic rciidiiig micro-ammeter A, or thc mirror-niiiinctcr It. Thc iiiirror reflects il spot, of light 011 to photoscnsi t h e , electrocnrdiogrnpliic pnpcr, rotating i l l tlic film cnsettc of t,he recording system (FC).
When 11 test tulle (Tt) is inscrtcd in the water I)ritIi, tlic \wyittg pllotoclcctric \ d u e s during ii progressive process, t,iiking pliicc i n tlie test tube, will be recorded iiutoiiiiiticnlly oil the film DilYcrciit tritcings, chitructcristic of the dill'crciit proccsws, will indicate tfhc progression of the rciictions.
Tlie stribility iiiitl reliability of iiiiy tcclinical itrriltlgctileiit, biiscd 011 the pliotoclcctric principle is known to be tlepciidcnt 011 iL iiuinbcr of fiictors. Tlicsc fiictors cnnnot lw discussed in dctiiil ill the present pitper, but iittentioti is dniwii to the following irnportiiiit points : (1) siliiill Huctuiitions in tlie curretit output due to teinpcriiturc viiriiitioiis, (2) the elenient of fittigiie, coninioiily not, w r y proiiouticctt in tho inoderii biirricr hyor cells (Weston pliot8ronic: cclls), (3) liig period of tlic ccll, rind