Finally, this came in from Kevin Bjorke, in response to my puzzlement over the glass globes that sometimes appear in pictorialist photographs:
The glass globe is an iconic reference to Dutch portraiture of the 1500s and 1600s, in which reference to the new optics industry indicated an interest in things new and scientific. It also indicated an interest in similar issues by the portraitist, a fact which has been construed by some (most famously David Hockney) to suggest that artists around this time accepted optical tools into their workshops, which in turn led to the strong explosion of "realistic" painting at that time (spreading from Holland to the rest of Europe).
Also, the globe had powerful social significance, because it indicated that the power of being iconified yourself — of being the subject of a portrait — was moving away from the ultra-powerful church and state authorities (saints, kings, and nobles of various sorts) and into the sphere of growing economic power — a connection to the optics industry usually meant a connection with telescope, navigation, commerce, shipping, and money. So we see it as an emblem of the rise of the merchant class.
The pictorialists (most infamously William Mortenson) just picked up all of the trappings left behind by those older traditions. Whether they were really aware of the meaning of such trappings is left open to question. But to a degree, they did — they were attempting, as the earlier painters had, to imbue their subjects (and themselves) with an illusory connection to power and substance.
— KEVIN BJORKE