Objective-C to Swift: NSScreen - objective-c

Question

I'm learning Swift. As a test, I'm translating some of my old Objective-C programs to swift. But I have a crazy error: In Objective-C I have the following code:
- (CGSize)makeSizeFromCentimetersWidth: (CGFloat)width andY: (CGFloat)height {
NSScreen *screen = [NSScreen mainScreen];
NSDictionary *description = [screen deviceDescription];
NSSize displayPixelSize = [[description objectForKey:NSDeviceSize] sizeValue];
CGSize displayPhysicalSize = CGDisplayScreenSize([[description objectForKey:#"NSScreenNumber"] unsignedIntValue]);
CGFloat resolution = (displayPixelSize.width / displayPhysicalSize.width) * 25.4f;
CGFloat pixelsWidth = 0.394 * width * resolution;
CGFloat pixelsHeight = 0.394 * height * resolution;
return CGSizeMake(pixelsWidth, pixelsHeight);
}
In swift I have translated to this:
func makeSizeFromCentimeters(width: CGFloat, height: CGFloat) -> CGSize {
var screen: NSScreen = NSScreen.mainScreen()!
var description: NSDictionary = screen.deviceDescription
var displayPixelSize: NSSize = description.objectForKey(NSDeviceSize)!.sizeValue
var displayPhysicalSize: CGSize = CGDisplayScreenSize(description.objectForKey("NSScreenNumber")!.unsignedIntValue)
var resolution = (displayPixelSize.width / displayPhysicalSize.width) * 25.4
var pixelsWidth: CGFloat = 0.394 * width * resolution
var pixelsHeight: CGFloat = 0.394 * height * resolution
return CGSizeMake(pixelsWidth, pixelsHeight)
}
In Objective-C the code does what it should: Calculate a size from centimeters to pixels, to give out (in my case) an NSImageView with exactly the size of the given centimeters. But in Swift, the returned size, is always 0:
NSLog("%f", makeSizeFromCentimeters(2, height: 2).width)
NSLog("%f", makeSizeFromCentimeters(2, height: 2).height)
Is there an translating error? Which variable is 0? (No idea why it should be 0 if it's not caused by a variable).
Thank you for your help!