This invention relates to injection devices. 
It concerns devices which are fitted with a syringe having a capsule with a needle projecting from the forward end and a plunger from the rear end.  A trigger and spring mechanism, when operated, shoots the syringe forwardly to project the needle, and then continues to act on the plunger to eject the dose.  There is also a return spring arrangement to retract the needle after use, thereby making it safe. 
In WO 99/03529 we have described such a device in which the trigger is arranges to provide an obstruction to the syringe, after its needle has been projected to prevent the syringe being retracted until the trigger is released or operated in another manner.  Then the return spring can act. 
This works well enough, but there is a potential problem with the device if it is used with a syringe with a low volume of prefill.  The total movement of the plunger can then be insufficient to allow the differential between the drive and return springs to permit the syringe to retract satisfactorily. 
It is the aim of this invention to ensure a more certain retraction. 
According to the present invention there is provided an injection device for containing and operating a syringe having a capsule with a needle projecting from the forward end and a plunger from the rear end, the device having a barrel enclosing the syringe and first and second springs initially held by a trigger assembly carried by the barrel in compressed energised states respectively at the rear and towards the forward ends of the barrel, wherein a first operation of the trigger assembly releases the first spring but keeps the second spring compressed so that the first spring urges the syringe forwardly by acting on the plunger and thence through the dose within the syringe, and then, when the syringe reaches a needle projecting position, presses the plunger forwards relative to the capsule to eject the dose, and wherein a second, different operation of the trigger assembly releases the second spring which exerts itself to retract the syringe and its needle. 
Preferably means will be provided for preventing the second operation of the trigger assembly before the first operation. 
The first, drive spring is conveniently housed in a cylindrical slider and acts between the rear end of the barrel and the forward end of the slider. 
Preferably the second, return spring is housed in a cylindrical carrier surrounding the capsule and acts between the rear end of the carrier and an abutment forward of the carrier provided by the barrel.  This can locate in the barrel and provide a guide for the syringe as well as containment for the return spring. 
The trigger assembly is conveniently a single rocker aligned longitudinally of the barrel, its rear end co-operating with the slider, to release that when its forward end is pressed in as said first operation, and its forward end co-operating with the carrier, to release that when its rear end is pressed in as said second operation.  The co-operation of the rear end of the rocker with the slider can be the agency preventing that rear end being pressed in until after the slider has moved the syringe to its needle projecting position. 
Since the return spring is out of commission until the injection has been carried out, a third, light spring may be provided to keep the syringe retracted against a stop until the device is fired.  This will present no significant resistance to the main drive spring when that is released. 
This containment of the return spring until it is actually needed means that it can be powerful enough to ensure retraction whatever the prefill and the amount the main drive spring hogan expanded.