IT seems almost cruel to recycle that hoary old Dickensian line about the best of times and the worst of times as Edinburgh and Glasgow prepare to add another chapter to Scotland's tale of two rugby cities.
After all, rarely in 142 years of matches between the two sides have their fortunes diverged quite so sharply.
When Gregor Townsend's Glasgow side run out at Scotstoun this evening, they will do so in the knowledge that a win will move them into second place in the RaboDirect PRO12 table and that the home play-off which has eluded them in previous seasons will be tantalisingly close to being secured. For Edinburgh, though, the mission is only to salvage some self-respect at the tail end of a campaign in which that quality has been apparent all too rarely.
There is some irony in the fact that one of the few games in which Edinburgh seemed energised was against Glasgow at Murrayfield last December. It was a time when the capital side seemed finally to have found some form - they had beaten Gloucester and Leinster in their two previous matches - but it was oh-so-typical of their season as a whole that, having dominated for more than an hour, they coughed up a soft try to Stuart Hogg and ended up losing 20-16.
It was doubly ironic that Glasgow were in the throes of a mid-season slump at the time. If Edinburgh considered that result an opportunity lost, their frustrations only deepened a week later when the return match at Scotstoun was called off because of a waterlogged pitch. Goodness only knows what would have happened had they been able to gain revenge that day but, in the interval between the postponement and today's rescheduled game, their trajectory has been steadily down, while Glasgow have headed off in the opposite direction.
The argument, of course, is that anything can happen in a derby. But the reality is that Edinburgh have failed to chalk up a single win in Glasgow since 2003. That record looks daunting enough, but the fact that Glasgow are on a run of five consecutive PRO12 wins, and have every incentive to stretch it to six, tilts the odds even further against the men from the east.
Yet Alan Solomons, the Edinburgh coach, looked far from chastened as he sat down to talk about the game yesterday afternoon. "It's certainly going to be a tough match," said the South African, with a smile. "Glasgow are riding high at the moment and they have had some really good wins. They are challenging for a top spot in the table and we appreciate that it will be a big challenge for us.
"But we said at the beginning of the season that we wanted to earn respect, and I think we have done that. That's very important as far as these last few games are concerned. We're playing three sides [their other games are at home to Munster and away to Leinster] who are all competing for the top spot. We can see the finishing line and we need to lift ourselves and deliver good performances."
Of course, Glasgow supporters will point to the fact that the first leg of the 1872 Cup was played on a quagmire pitch at Murrayfield and that their side's recent improvement can be linked to faster, firmer tracks they have enjoyed in recent weeks. Certainly, Glasgow have found tries easier to come by now that winter has made way for spring. They picked up their first try-bonus point of the season against Treviso three weeks ago and then touched down three times in each of their next two outings, against Munster and Ulster.
Does Solomons expect tonight's opponents to come flying out of the blocks? Not exactly. "There is huge motivation for them to come away with a bonus point win," he added. "But I think they have a reasonably structured style and I don't think they will deviate a lot from that. There will be a bit of movement, but I think they will look to build their innings. We've just got to be prepared for any eventuality."
That consideration was surely a factor in the return of Greig Laidlaw to the starting line-up. With a lucrative contract with Glouceter freshly signed and tucked away in his back pocket, the little scrum-half might have been forgiven for taking his eye off the ball as he sees out his final days with Edinburgh, but Solomons clearly recognised the value of the 28-year-old's level-headed rugby nous in a game where tempers and old rivalries tend to lend a fractious edge to the occasion.
So, too, did hooker Ross Ford when he spoke of the qualities of his fellow Borderer. "Greig has captained Scotland on numerous occasions and Edinburgh for just about forever," he said. "He has that aura about him on the pitch so that when things aren't quite going your way, or if you are getting carried away, he'll bring the team back to an even keel.
"He's a very clever player. He puts pressure on teams in the way he distributes the ball and his kicking game is very good. He sees things clearly and can exert pressure that way."
With Al Kellock back in their ranks, Glasgow are not exactly short of inspiration either. Even Solomons described the giant lock as a "talismanic" figure. Kellock relishes the inter-city derbies like few other players and it is easy to suspect that the born-and-bred Glaswegian is trying to pay penance for the four seasons he spent on Edinburgh's books before his move west in 2006.
In the evening of his career and with three PRO12 play-offs - all away, all lost - behind him, Kellock is determined to give Glasgow their best chance of glory this season. Ford, though, has other ideas.
What, Ford was asked, did he think of the prospect of spoiling Glasgow's party? A conspiratorial grin crossed his face. "Yeah," he replied. "That would be good."