In The Great Gatsby, explain why "the colossal significance of that light had now vanished forever" and why Gatsby's "count of enchanted objects had diminished by one."
Nick Carraway makes both of these observations after witnessing Gatsby's long desired reencounter with Daisy. Both observations point to the idea that no matter how wonderful reality is, it can never match our dreams.
For five years, Gatsby's chief motivation has been reuniting with Daisy. It has guided everything he has done, including amassing a fortune to impress her, giving huge parties in the hopes she will turn up among the guests, and buying a mansion across the water from her own to be close to her. Every night, Gatsby has been watching the green light, dreaming of seeing Daisy again. Now that he has realized the dream, the light can't possibly mean as much to him. With gain comes loss. Achieving reality means losing the sweetness of the dream.
Likewise, Daisy herself is reality, not a dream. Nick, a witness to this colossal event of the two meeting again, perceives that even though Daisy is wonderful and charming with Gatsby, no human can ever live up to another person's fantasy version of them. As Daisy moves from "enchanted"—i.e., Gatsby's idealized dream woman—to reality, she inevitably loses some of the sparkle Gatsby's fantasies of her invented. Nick hits one of his more lyrical heights—and foreshadows Gatsby's death with the word "ghostly"—as he observes:
There must have been moments even that afternoon when Daisy tumbled short of his dreams not through her own fault, but because of the colossal vitality of his illusion. It had gone beyond her, beyond everything. He had thrown himself into it with a creative passion, adding to it all the time, decking it out with every bright feather that drifted his way. No amount of fire or freshness can challenge what a man will store up in his ghostly heart.
Where the green light had once stood for Gatsby's dream and his hope to achieve the ideal marriage with Daisy, at the end of the novel this is no longer possible.
As hope is an essential component of Gatsby's character, the light that represents his hope attains a "colossal significance" for Gatsby and for the novel. Nick repeatedly aligns Gatsby with this hope and a curiously naive idealism, even in his first descriptions of Gatsby.
"If personality is an unbroken series of successful gestures, then there was something gorgeous about him, some heightened sensitivity to the promises of life.… [Gatsby had] an extraordinary gift for hope, a romantic readiness such as I have never found in any other person and which it is not likely I shall ever find again."
Gatsby's view of the green light across the bay from his house is one of enchantment because Gatsby is thoroughly taken up with the idea of fulfilling his ideals and his dreams, which are focused on marrying Daisy. Her house is on the other side of the bay, near the light. When she retreats into that house after running down Myrtle in a car accident, Daisy goes beyond Gatbsy's reach. The enchantment ends.
Daisy has given up on marrying Gatsby. She seems to give up during the confrontation episode where Tom and Gatsby each make claims on Daisy's love.
In this episode Gatsby demands that Daisy say she never loved Tom (which she refuses to say) and Tom reveals a number of tawdry rumors about Gatsby's underworld connections. Daisy is disappointed and upset by this episode and Gatsby's hope to win her over end.
This quote is taken from chapter 5 of The Great Gatsby. To put it into context, the light refers to the green light which glows from across the water, on the dock where Daisy lives. For years, Gatsby has gazed at that light, longing to be with Daisy. The light, therefore, symbolized his desire to be reunited with her, to live his version of the American Dream.
By chapter 5, Gatsby has indeed been reunited with Daisy. The green light, therefore, no longer has the same symbolic meaning for Gatsby because he has achieved his dream of being with Daisy. This explains why the light has "vanished."
Similarly, his "count of enchanted objects" has decreased by one because Daisy is with him. She was one aspect of his dream. Meeting up with her was a treasured "object." Now, his dream shifts to some of the other "enchanted objects:" specifically, marrying Daisy and spending the rest of his life with her.
Gatsby will, however, have to overcome Tom if he is to make that happen.