For Apple's earnings, it's all eyes on the iPhone

Apple reports its third-quarter results tomorrow, and Wall Street's looking at the iPhone for a bright spot.

Apple announces its third quarter earnings on Tuesday, and -- as usual -- all eyes are on how well the iPhone's selling.

The hit product has become a key part of Apple's business and the fuel to dramatic growth over the past several years, though that growth has slowed.

Reports from some phone carriers have provided a peek at Tuesday's results, specifically Verizon, which reported iPhone sales last week that were more than the carrier originally expected.

Wall Street is expecting Apple to post earnings of $7.31 per share on sales of $35 billion. Those estimates are on the higher end of how much Apple said it expected to bring in when it reported its last quarter back in April -- a range of $33.5 to $35.5 billion in sales.

In either case, that's down from earnings of $9.32 per share, and flat with the $35 billion in sales Apple reported the same quarter a year ago.

One of the key reasons for a dip in the amount of profit Apple's making are gross margins, or how much Apple's making on the products it sells. By Apple's forecast, those are expected to be between 36 and 37 percent, down from the nearly 43 percent from the same quarter a year ago.

During an earnings call earlier this year, Apple noted that some of the change in its margins was due to the company making less on the iPad Mini than some of its other popular products. It also attributed some to a weaker U.S. dollar, adding that things could improve as the company brings down costs.

Wall Street's expecting Apple's sales to work out to around 26.5 million iPhones, which would be half a million more than the company sold during the same time last year. Similarly, Apple's expected to sell 18 million iPads, around 1 million more than last year, though down from the 19.5 it sold last quarter. Other estimates include sales of 3.9 million Macs (versus last year's 4 million), and 4.8 million iPods (down from last year's 6.8 million).

For the next quarter, Wall St. analysts polled by Thomson First Call are expecting Apple to report earnings of $8.16 per share on $37.8 billion in sales, with a margin of 36.8 percent. That would be down from $8.67 profit per share on $36 billion in sales and 40 percent gross margin the company made during the same quarter last year.

Apple will report just after the market closes on Tuesday, followed by a conference call with executives at 2 p.m. PT. CNET will have all the relevant news from both, so stay tuned.