When we meet Eustacia, we learn that she's carrying two rather unusual items with her: an hourglass and a telescope.
For the rest, she suffered much from depression of spirits, and took slow walks to recover them, in which she carried her grandfather's telescope and her grandmother's hour-glass – the latter because of a peculiar pleasure she derived from watching a material representation of time's gradual slide away. (1.7.22)
So what's up with these two objects? Well, these two symbolic objects represent two of the books major themes and help us explore Eustacia's relationship with these themes. These objects stand for time and sight.
Let's start off with the hourglass. Eustacia gets a weird kick out of watching time physically slide away, perhaps because it means that yet another terrible day on the heath is over for her. But what else is going on with this symbol?
[T]he middle article being the old hour-glass, and the other two a pair of ancient British urns which ha been dug from a barrow near, and were used as flower-pots for two razor-leaved cactuses. (2.4.4)
The hourglass here is surrounded by old, historic objects – ancient urns and cacti that seem like prehistoric plant life. But why is the hourglass connected to the past? Well, an hourglass is a rather old-fashioned way to tell time – there were clocks back in Hardy's day. In fact, Victorians really loved their new-fangled clocks. Time wasn't measured by the natural world as much anymore and cities began to run on different sorts of time than the countryside still did. (Check out this cool site to learn more about the history of how people told time.)
And Eustacia isn't so divorced from the modern world that she relies solely on an hourglass to tell time; she actually carries a watch with her (1.6.13). The fact that she still favors using an hourglass gives us some insight into how Eustacia approaches time.
First, Eustacia's use of an hourglass points to her situation on the heath – an old-fashioned and isolated place. Ironically, Eustacia longs to escape the heath, yet she definitely has an old-fashioned vibe. So Eustacia's approach to time may point to her counter-cultural tendencies and her penchant for going against the crowd. Likewise, as we noted earlier, the hourglass's physical representation of time passing appeals to Eustacia's sad and thoughtful nature; she likes to dwell on things, and the hourglass really lets her dwell on time's passage.
The telescope is a bit more obvious than the hourglass. After all, Eustacia uses it to spy on people, so the telescope's ties to the theme of sight are pretty straightforward. However, this idea of spying gives us further insight into Eustacia's character.
She lifted her left hand, which held a closed telescope. This she rapidly extended, as if she were well accustomed to the operation, and raising it to her eye directed it towards the light beaming from the inn. (1.6.10)
Eustacia tends to set herself apart from people – she eavesdrops and crashes a party in disguise before finally meeting Clym (the object of her stalking) face-to-face. Eustacia often does things in a very roundabout way and the telescope points to her tendencies here. She spies on people, she engages in subterfuge and secret meetings, and the girl likes to be sneaky. All these things indicate a very imaginative and romantic personality, which ties back to Eustacia's tendency to confuse fantasy and reality. The things Eustacia sees aren't always reality and her fondness for viewing stuff via a telescope point to her fondness for "seeing" things as romantic when there might not be any there.