Theglasstubebeingrevolvedonitscentre，theindexissettothenumberofhalfsecondsrequired，andthesandrunningdown，therequiredtimeismarkedwithoutthepossibilityoferror。
Inpracticeitwillbefoundtobeafarmoreconvenientinstrumentforthepurposethaneitheraclockorasecondswatch，andisapplicablebothforthecameraandmercurybox。\"
Iftheartistfindsitdesirableornecessarytotaketheobjecttobecopiedinitsrightposition，thatisreversetheimageonthespectrum，hecandosobyattachingamirror（whichmaybehadofMr。Anthony，orMr。Roach）tothecameratube，atanangleofforty-fivedegrees。
If，aftertakingtheplatefromthecamera，itbeexamined，nopicturewillyetbevisible，butthisisbroughtaboutbytheFOURTHPROCESS。——BringingoutthePicture，orrenderingitVisible。——
Wenowcometotheuseofthemercurybath，Fig。11。Tothebathathermometerisattached，toindicatetheproperdegreeofbeatrequired，whichshouldneverberaisedabove170deg。Fahrenheit。Theplatemaybeputintooneoftheframes（seeFig。11，）overthemercury，facedownwards，andexaminedfromtimetotime，bysimplyraisingitwiththefingers，orapairofplyers。Thisoperation，aswellastheothers，shouldtakeplaceinthedarkcloset。